Enhancing Selectivity, Sensitivity, and Molecular Identification ......2021/01/28 · 2020-063788...

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Enhancing Selectivity, Sensitivity, and Molecular Identification towards High-performance Graphene Gas Sensors

Materials Science Graduate School of Advanced Science and Technology

Mizuta-Manoharan Laboratory Osazuwa Gabriel Agbonlahor (s1820001)

1. Background Detecting minute concentrations of gaseous pollutants in the atmosphere and human breath enables environmental monitoring and non-invasive detection of ailments. Consequently, due to graphene’s single-molecule sensitivity, it is highly sought after for high sensitivity gas detection. However, its poor selectivity, huge p-doping in the atmosphere, and inability to identify adsorbed gases makes graphene less useful for practical applications in environmental and clinical gas sensors which are typically deployed in atmospheric conditions.

2. Aim In this work, we aim to develop high-performance CVD graphene-based gas sensors suitable for environmental monitoring and non-invasive detection of ailments. To this end, we demonstrate a facile and viable route for the development of an ammonia selective activated carbon functionalized graphene field-effect transistor (a-CF-GFET) which shows minimal atmospheric induced doping. The a-CF-GFET gas sensor demonstrated good recoverability, extreme ammonia sensitivity (500 ppt) in atmospheric air, with ~ 3 seconds response time. Furthermore, retention of gas adsorption induced charge transfer in the graphene-gas molecule van der Waals complexes is demonstrated. Hence, a proof-of-concept (the CNPD method) for characterizing adsorbates on the graphene channel is achieved and new insights on the graphene-gas van der Waals bonding interaction realized.

3. Research Results Herein, an activated carbon-functionalized graphene sensor with extreme ammonia

sensitivity and selectivity in atmospheric air is developed (Figure 1a) from the pyrolysis of the of the electron beam cross-linked Novolac resin on the graphene nanoribbons (Figure 1b). We show that the activated carbon (a-C) on the graphene channel is oxidized and porous (Figures 1c&d), consequently, parts per billion concentrations of ammonia (Figure 2) down to 500 parts per trillion (ppt) were detected in the atmosphere with a response time of 3 seconds making the a-CF-GFET sensor the most sensitive ammonia selective sensor so far reported. The extreme ammonia sensitivity makes the a-CF-GFET sensor suitable for environmental monitoring and non-invasive medical diagnosis of ailments such as ulcers and kidney problems using ammonia as a biomarker.

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Figure 1. Morphological characteristics of the a-CF-GFET sensor. (a) Device schematics of the a-CF-GFET sensor showing porous a-C on graphene (left). Inset is the zoomed in image of the pore circled in red. The Raman spectra of the a-CF-GFET sensor showing a broad a-C peak is shown on the right. (b) AFM image showing 38 nm a-C on graphene. (c) TEM image (bright field) showing the fibrous nature of the lithographically cross-linked a-C on the graphene (left), and the more amorphous uncrosslinked a-C on graphene (right). Dark regions are activated carbon, while bright regions are pores leading to graphene. Scale bars in both images are 20 nm. (d) EDX mapping of oxygen around a-C pores on graphene (left) showing minimal oxygen in pores and more oxygen on the surrounding a-C. Top-right is the EDX carbon map of the same region. The bottom right image is the dark field TEM image of the same a-C region on graphene used in EDX mapping. Bright regions are the activated carbon while the dark regions are the pores.

Figure 2. CNP shift of a-CF-GFET sensor in response to ammonia. (a) 500 ppt ammonia in atmospheric air, (b) 1 ppb in atmospheric air (c) 2 ppb ammonia in atmospheric air (d) 5 ppb ammonia in atmospheric air. (e) 200 ppb ammonia in atmospheric air.

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Next, we demonstrate that the electric field induced graphene-molecule charge transfer is retained in the graphene-molecule vdW complex even after the electric field is turned off, (Figure 3a) with the retained charge still unique to the applied electric field magnitude and direction. Consequently, the vdW bonding memory of adsorbed molecules on graphene was observed (Figure 3a, blue region). This bonding memory enabled the molecular identification of adsorbed gases on graphene using a proof of concept, the CNPD method (Figures 3b & c), which is a charge transfer based molecular identification technique that measures the gas adsorption induced charge-transfer at ∓tuning voltages (∓Vt).

Figure 3. (a) vdW doping concentration vs time for CO2 adsorption on graphene at Vt = 40 V, 36 V, 0 V, -20 V, -40 V: white region = tuning voltage applied region, blue region = no tuning voltage applied (vdW bonding memory region). (b) Experimental schematic for the CNPD method (left) and definition of the disparity value for consecutive - Vt and + Vt transfer characteristics measurements (right). (c) Dependence of the CNPD value of the a-CF-GFET device to various gas environments.

The difference between the –Vt and + Vt induced charge transfer called the disparity value (Figure 3b) obtained from the shift of graphene’s charge neutrality point was shown to be characteristic of various gas environments (Figure 3c), and dependent on the tuning time and applied tuning voltage.

4. Accomplishment Publication List

1. Agbonlahor, O.G., Muruganathan, M., Imamura, T., and Mizuta, H. Adsorbed Molecules as Interchangeable Dopants and Scatterers with a Van der Waals Bonding Memory in Graphene Sensors. ACS Sensors 5, 2003–2009 (2020).

2. Iwasaki, T., Nakamura, S., Agbonlahor, O. G., Muruganathan, M., Akabori, M., Morita, Y., Moriyama, S., Ogawa, S., Wakayama, Y., Mizuta, H., Nakaharai, S. Room-Temperature Negative Magnetoresistance of Helium-Ion-Irradiated Defective Graphene in the Strong Anderson Localization Regime. Carbon, 175, 87–92 (2020).

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3. Agbonlahor, O.G., Muruganathan, M., Ramaraj, S.G., Wang, Z., Hammam, A.M.M., Kareekunnan, A., Maki, H., Hattori, M., Shimomai, K., Mizuta, H. Nanopored Activated-Carbon Functionalized Graphene Molecular-Sieve for Atmospheric ppt-Level Gas Sensing and Molecular Identification. Advanced Materials (Under Review)

4. Agbonlahor, O.G., Muruganathan, M., Ramaraj, S.G., Taketomi, K., and Mizuta, H. Temperature and Pressure Effects on Gas Adsorption-Induced Molecular Dynamics in Activated Carbon Functionalized Graphene. (In preparation)

Patents 1. 'Sensor devices and their manufacturing method'. Manoharan Muruganathan, Gabriel

Agbonlahor, Amit Banerjee, Hiroshi Mizuta, and 4 other co-inventors. Filing number: 2020-063788 (Filing Date: 31 March 2020)

2. 'Sensor devices, their manufacturing method and gas identification method' Manoharan Muruganathan, Gabriel Agbonlahor, Amit Banerjee, Hiroshi Mizuta, and 4 other co-inventors. Filing number: 2020-063789 (Filing Date: 31 March 2020)

3. 'Gas identification method and gas identification system' Manoharan Muruganathan, Gabriel Agbonlahor, and four co-inventors Filing number: 2019-157961 (Filing Date: 30 August 2019)

Conferences

1. Adsorption Distance Effects on Gas Adsorption Induced Scattering in Graphene Gas Sensors. Osazuwa Gabriel Agbonlahor, Manoharan Muruganathan, Tomonori Imamura, Hisashi Maki, Masashi Hattori, Hiroshi Mizuta 2020 International Conference on Solid State Devices and Materials (SSDM2020), 27-30 Sept 2020, Toyama, Japan (Online).

2. Atmosphere-Specific Response in Graphene Field-Effect Transistors Osazuwa Gabriel Agbonlahor, Manoharan Muruganathan, Hiroshi Mizuta 2020 年 3 月 12‐15日、第 67 回応用物理学会春季学術講演会、上智大学四谷キャ

ンパス. 3. Detection of ammonia via charge transfer-induced shift of graphene charge neutrality

point. Osazuwa Gabriel Agbonlahor, Jothiramalingam Kulothungan, Manoharan Muruganathan , and Hiroshi Mizuta 第 79 回応用物理学会秋季学術講演, 2018 年 9 月 18 日-21 日、名古屋国際会議場.

Syntheses and Characterizations of Diketopiperazine Monomers and Polyamides

Doctor of Science (Materials Science)

Kaneko Laboratory, 1820004, Hongrong YIN

Part 1: Research Content

1. Background

Chirality and self-assembly are basic attribute of nature and closely related life activity.

Most of biopolymers, such as polypeptides, polynucleotides, and polysaccharides, and

their basic units are chiral. Also, the self-assembly of these biopolymers is important in

life activity, for example, double helix structure of DNA is significant for transcription.

In materials science, chirality and self-assembly of biopolymers also inspire the

development of functional materials, especially in cases of amino acids and polypeptides.

Amino acids are abundant in nature, and also can be produced from biomass. With various

side chains, amino acids can be easily modified, and applied in synthetic polypeptides.

Synthetic polypeptides have been used in in many fields, especially in medical

applications such as nanoparticles, drug delivery system, and tissue engineering.

2,5-Diketopiperazines (DKPs), which are simple cyclic dipeptides, are common in

nature or easily synthesized by the condensation of α-amino acids. Based on two

symmetrical amide groups in a six-membered ring, DKPs have four sites able to form

hydrogen bond, which result in highly molecular arrangement properties. Due to the

strong hydrogen bonding and chiral structure, DKPs have been studied in medicine design,

chiral catalyst, low molecular weight gelators, and annexing agent for polymers. However,

though DKPs have many interesting properties, only a few DKP-based polymers were

reported and the effects of DKP moiety on the corresponding properties of polymers are

not clear. Otherwise, the synthesis of DKP monomer is challenging and important for the

synthesis of polymers.

The aim of this work is to study chirality and self-assembly properties of DKP

monomers and resulting polyamides. This work is divided into three parts: (1) syntheses

and configurational studies of diketopiperazine stereoisomers; (2) syntheses and solvent-

controlled self-assembly of diketopiperazine-based polyamides from aspartame; (3)

syntheses and stereochemistry-property studies of diketopiperazine-based polyamides.

2. Syntheses and configurational studies of diketopiperazine stereoisomers

To synthesize AB-type DKP monomers, cyclo(aspartyl-4-amino-phenylalanyl)

(ADKP), step-wise protection and deprotection of (L/D)-aspartic acid and (L/D)-4-nitro

phenylalanine was performed (scheme 1). Caused by the solubility problem of precursors

of trans-ADKPs, LD- and DL-ADKP were synthesized in low yield. No racemization

occurred during the synthesis. Structural characterizations and studies of stereoisomers

were performed by 1H NMR, ROESY, FTIR, and CD. In DMSO solution, configurations

of cis- and trans-ADKP were confirmed, in which DKP ring was in planer structure, and

the folded benzene ring was stabilized by Cβ-H…π interaction in cis-structures and Cα-

H…π interaction in trans-structures. Self-assemble behavior of stereoisomer was studied

by solvent displacement method. LL- and DD-ADKP show similar chrysanthemum-like

morphology, while LD-ADKP shows rose-like morphology. The present study provides

a synthesis method for stereoisomers of DKP, and structural insight of phenylalanine-

aspartic acid-based DKP, which have potential for drug and catalyst design.

Scheme 1 Synthetic route of ADKP stereoisomers form protected aspartic acid and 4-

nitro phenylalanine, and structures of ADKP stereoisomers.

3. Syntheses and solvent-controlled self-assembly of diketopiperazine-based

polyamides from aspartame

LL-type ADKP was synthesized from aspartame and subsequently utilized in the

polycondensation of homo-polyamide (PA1) with high molecular weights. By using

various amino acid, dicarboxylic acid, and diamine, random DKP-based copolymers were

also synthesized. The self-assembly properties of ADKP and PA1 were studied via the

solvent displacement method. Notably, PA1 self-assembled into particles with various

morphologies in different solvent systems, such as irregular networks, ellipsoids, and

vesicles (Figure 1). The morphological transformation was also confirmed by dropping

acetone and toluene onto the PA1 particles. Furthermore, infrared spectra and Hansen

solubility parameters of PA1 and different solvents revealed the particle formation

mechanism, which provided more insights into the relationship between the morphology

and strength of the hydrogen bonding of each solvent.

Figure 1 SEM images of particles obtained by the dispersion of PA1 into (a) water, (b)

ethanol, (c) THF, (d) ethyl acetate, (e) acetone, and (f) toluene.

4. Syntheses and stereochemistry-property studies of diketopiperazine-based

polyamides

Homo-polyamides and co-polyamides from ADKP stereoisomers were synthesized.

Due to same chemical structure, all PAs showed similar molecular weight, thermal

properties and solubility. Determined by CD spectroscopy, all PAs showed optical activity.

Moreover, solvent/third molecule effect on LL-PA was investigated by adding other

solvents into the LL-PA DMSO solution. When water and ethanol were added, LL-PA in

DMSO became optical inactivity. It is suggested that water or ethanol disturbed hydrogen

bond between DKP units in polymer chain, which played an important role in stabilizing

secondary structure of polymer. Self-assembly of PAs were investigated. LL-PA, DD-PA

and LLcoDD-PA self-assembled into vesicles, while LD-PA self-assembled into cubic

structure in toluene and acetone (Figure 2). The present study provides structural insights

of DKP-based polymers with stereochemistry, and reveals their optical and self-assemble

properties.

Figure 2 SEM images of particles obtained by (a) LL-PA in acetone, (b) LL-PA in

toluene (c) DD-PA in acetone, (d) DD-PA in toluene, (e) LLcoDD-PA in acetone, (f)

LLcoDD-PA in toluene, (g) LD-PA in acetone, and (h) LD-PA in toluene.

Part 2: Research Purpose

DKP unit is an important building block for medicines, catalysts, supramolecules, and

has enormous potential for development of functional materials. However, polymers

carrying DKP unit in backbone have not been well explored and are lack of study. In this

study, novel AB-type DKP monomers and relative polyamides with stereochemistry were

successfully synthesized. Chirality and self-assembly properties of obtained monomers

and polyamides were well studied. This study creates a new insight on stereochemistry

and self-assembly of DKP monomers and polymers, which could lead to the development

of medicines and functional materials.

Part 3: Research Accomplishment

Synthesis and solvent-controlled self-assembly of diketopiperazine-based polyamides

from aspartame. Yin, Hongrong; Takada, Kenji; Kumar, Amit; Kongprathet, Thawinda

Kaneko, Tatsuo. RSC Advances.

Title: In-situ scanning electron microscopy observation of electrode-electrolyte interfaces in

an electrochemical cell

Intended degree: Doctor of Science (Materials Science) Laboratory: Tomitori Laboratory Student No.: 1820008 Name: He Gada Part 1: Research Content

Understanding of the dendritic structure growth and stripping mechanism during electrochemical

reactions is of great importance to improve the functionality of electrochemical technique devices.

Thus, a lot of devoted works had been conducted to uncover the mechanisms qualitatively and

quantitatively through combination of typical electrochemical measurements using macro scale

electrodes. As an imaging technique during electrochemical reactions, electron microscopies with high

resolution meet the desire for both the local structure of electrode and the dendrite formation at

nanometer level characterization. For most of the previous works, transmission electron microscopy

(TEM) was utilized for in-situ real time observation of metallic crystallization inside the liquid

specimen. There have been difficulties like damages due to electron beam irradiation at thigh electric

current densities and preparation of liquid specimen with thicknesses of thinner than 100 nm, which

is the limitation of transmission of electrons in TEM, and so on. As for the in-situ pseudo real-time

scanning electron microscopy (SEM) observation is more practical than TEM observation, because of

relatively low current density and no particular requirement for the thickness of the specimen.

We developed a conventional in-situ electrochemical cell (Figure 1) with two electrode terminals

for SEM to observe the processes of electro-plating and stripping simultaneously with measuring the

cyclic voltammetry.

The cell was sized to fit into the small space of the custom-made specimen holder for Hitachi S-

5200 Fe-SEM with

several current

feedthroughs, each

of which was

connected to a

coaxial cable. The

front plane of the

cell-body

possessed a drain

on the side of an

insulator plate with

Figure 1. (A) Schematic illustration of the exploded view of

electrochemical cell. (B) B-1: cell-body, B-2: TEM grid with vapor

deposited Au electrode, B-3: cell-cover with O-rings, B-4: form of cell-

body and PEEK insulator is installed into SEM holder.

two holes for the cables, as well as a vessel for liquid electrolyte at the center of the insulator plate.

The insulator had a circular depressed

seat for a silicon nitrite TEM grid. Two

Au wires inserted into Kapton tubes were

connected to the two coaxial cables

respectively, and inserted through the

wire-holes of cell body and insulator;

only the Au wires were set to the

concentric hole and connected to two

vapor-deposited Au electrodes on the one

side of the TEM grid. There were two

concentric circular seats for O-rings on a

cell cover; one was smaller than the

silicon nitrate grid and the other one was

larger than that. Care was taken so as not

for electrolyte to leak to the vacuum

environment during SEM experiments.

When the applied potential was varied,

electrochemical reactions occurred at the

end of the vapor-deposited electrode and

were observed through a small thin

silicon nitrite membrane window of TEM

grid with a thickness of 50 nm, through

which the SEM electron beam was able

to pass.

After inserting the SEM specimen holder

assembled with the electrochemical cell into

the SEM, the working electrode and the

counter electrode of the two Au electrodes

were electrically connected to the inputs of

a potentiostat/galvanostat instrument

(VersaSTAT4, Princeton Applied Research);

the inputs for the counter and reference

electrodes were connected each together for

simple electrochemical measurements. We

visualized the electro-plating and stripping

Fig. 2. (a) First shot of time lapse SEM image of

the Au electrode in the CV measurement. The square

region, denoted in red, was displayed in the

following SEM images in (b)–(f). The full video of

the SEM images with a view area of ~100 µm × 100

µm is available at Supplementary data I. (g) Time

evolution chart of the potential and current

measured with the SEM images in (a)–(f). The

broken lines indicate the times when the respective

SEM images were started to be taken for 8.4 sec. (h)

Cyclic voltammogram plotted from (g).

of Pb dendrites on the Au electrode in an electrolyte of 1.5 M Pb(NO3)2 aqueous solution via in-situ

SEM observation (Figure 2).

The bias voltage was swept at a rate of 50 mV/s in a triangular waveform in the range from +2V to

–2 V between the working and counter/reference electrodes. First, no obvious reaction was observed

when the bias voltage decreased from 2 V to 0 V. From 0 V, small islands started to form at the

electrode-electrolyte interface and turned into dendritic structures gradually, there was no obvious

change to the measured current until the voltage reached –1.25 V. The dendritic structures rapidly grew

with increasing current from –1.25 V, and the current peak appeared at –2 V. After the direction of

sweeping voltage was reversed, the dendritic structures drastically dissolved with decreasing current

up to –1.25 V. Afterward, the current showed no obvious change whereas the dendritic structure

dissolved completely around 1.5 V. With increase in the sweeping rate, the peak value of the current

and the redox potential difference increased. When the concentration and composition of electrolyte

were changed, the reduction potential shifted toward the negative voltage direction and the oxidation

potential shifted toward the positive direction. We observed in-situ consecutive SEM images of electro-deposition and stripping of Pb on the Au

electrode through the SiN membrane window while measuring the cyclic voltammogram. The SEM images

clearly showed that the electro-deposition and stripping of Pb on a nanoscale, which were consistent with

the cyclic voltammogram curves. From the SEM observation, we distinguish the growth modes of Pd

deposits, exhibiting the particulate band structures and dendrite strucutres, depending on the roughness of

the electrode. Also the brightness of SEM images in the electrolyte region changed in the course of the

cyclic voltammetry, which implied the change of Pb concentration of the electrolyte near the electrode. This

study demonstrated the potential of in-situ high-resolution SEM combined with the electrochemical cell.

Part 2: Research Purpose

Most of the previous work for in-situ TEM observation, the current density must be very high when

taking high resolution TEM images, which causes H2 bubbling in the liquid electrolyte due to electron

beam irradiation. Practically, magnification should be reduced to avoid such irradiation damage, since

the current density depends on the magnification. In this sense, I think that in-situ SEM observation

seems to be better than TEM, since the current density is relatively low. The sample preparation for

in-situ TEM observation of specimens in liquid is time consuming and challenging. The thickness of

liquid specimens has to be thinner than 100 nm in order to achieve high spatial resolution imaging. As

for the in-situ SEM observation, there is no such requirement for a thin specimen. As concerned with

industrial application, nanomaterials such as nanoparticles or nanowires with around 100 nm in size

have been synthesized mainly by electrochemical methods. The spatial resolution of SEM is high

enough to investigate the electrochemical reaction for nanomaterials of such a size. The equipment for

in-situ SEM observation has been commercialized by some companies, but they have some

disadvantage for laboratory research such as high financial cost, experimental limitation and so on.

Dendritic structure would appear in electrochemical technique devices like lithium ion battery and fuel

cell, it would determine the power density, lifetime of these devices negatively. Through this work,

intend to provide a practical experimental method to study the energy storage devices in industry.

Part 3: Research Accomplishment

Submitted paper:

He, Gada; Oshima, Yoshifumi; Tomitori, Masahiko

In-situ high-resolution scanning electron microscopy observation of electro-deposition and stripping

of lead in an electrochemical cell

Japanese Journal of Applied Physics

Conferences:

1. 14th International conference on Atomically Controlled Surfaces, Interfaces and Nanostructures

(ACSIN-14) Dates: October 21 – 25, 2018

Venue: Sendai International Center, Sendai, Japan

Oral presentation: In-situ scanning electron microscopy observation of electro-plating and stripping

of lead dendrites in an electrochemical cell

2. The 66th Japan Society of Applied Physics Spring Meeting, 2019

Dates: March 9 – 12, 2019

Venue: Tokyo Institute of Technology, Ookayama Campus, Tokyo, Japan

Oral presentation: Electro-plating and stripping of lead dendrites observed by in-situ scanning electron

microscopy with an electrochemical cell

3. The 80th Japan Society of Applied Physics Autumn Meeting, 2019

Dates: September 18 – 21, 2019

Venue: Sapporo Campus, Hokkaido University, Sapporo, Japan

Oral presentation: In-situ scanning electron microscopy observation of lead dendrites grown in an

electrochemical cell

4. Annual Meeting of the Japan Society of Vacuum and Surface Science 2019

Dates: October 28 – 30, 2019

Venue: Tsukuba International Congress Center, Tsukuba, Japan

Oral presentation: In-situ scanning electron microscopy observation of electrode–electrolyte interfaces

in an electrochemical cell

Abstract

A study of atomic scale mechanics by in situ transmission electron microscopy with a quartz

length-extension resonator

Zhang Jiaqi (s1820021) Oshima Lab

Doctor of Science (Materials Science)

1. Research Content

Background

The mechanical properties of atomic-scale materials are novel not only for development of

nanoelectromechanical systems (NEMS) such as sensors but also for development of novel

functional devices that utilize the fact that the physical and chemical properties of nanocontacts are

sensitive to structure and shape. It has been mainly investigated by theoretical calculation, and the

size effect or crystal orientation dependence have been pointed out such that Young’s modulus

depends on the size due to surface effect below 10 nm in size. However, such dependence has not

been clarified in the experiment. However, no consensus has been obtained due to problems such as

different results being obtained depending on the pseudopotential used in the calculation.

Aim

In this thesis, I developed a TEM holder incorporating a length extension resonator (LER) and

applied frequency modulation method to the LER in order to directly measure spring constants of

nanomaterials with high accuracy. Fig. 1 (A) is an overview view of the developed holder. Since the

LER has a high Q factor, the amplitude of the oscillator required to measure the equivalent spring

constant is only about 20 pm, which is a big advantage because the plastic deformation of the

measured nanomaterial can be avoided and atomic resolution image can be obtained. Therefore, it

is possible to measure the equivalent spring constant of the metal nanocontact with high accuracy.

Fig. 1 (B) shows time evolution of electrical conductance and equivalent spring constant of a Pt

nanocontact in the process of slowly stretching it until breaking, and TEM images taken from the

video.

As mentioned above, the structure, conductance, and equivalent spring constant of nanocontacts

could be obtained using the developed TEM holder. From these data, we aim to investigate the

mechanical properties of the nanomaterials such as the Young’s modulus.

Figure 1. (A) design of the TEM holder, (B) Conductance and spring constant trace and snapshot

TEM images during the stretching process of a Pt nanocontact.

Experimental results

Using our developed holder, we precisely measured the spring constant (stiffness) of Pt

monatomic chains have 2-5 atoms as shown in figure 2(A). And the individual bond stiffness in tip-

suspended platinum (Pt) monatomic chains was determined by statistically analyzing the stiffness

of about 150 Pt monatomic chains consisting of 2–5 atoms. The single bond stiffness at the middle

of chain was estimated to be 25 N/m, while that of the bond to the suspending tip was 23 N/m. These

stiffness values are obviously higher than the bulk counterpart of 20 N/m. The chain spring kept the

constant stiffness during stretching process, similarly to the quantized conductance. In addition, the

bond length at equilibrium and at maximum breaking point has been estimated to be 0.25 nm and

0.31 nm, respectively, in the monatomic chain. It indicates that the maximum elastic strain of one

bond in the chain is as large as 24%. These characteristics including the breaking point could be

explained by the configurations minimizing the calculated string tension.

In addition, in order to clarify the size effect on the Young’s modulus, the Au nano junctions

with the axis of the [111] direction was evaluated below 3 nm in diameter by our developed TEM

holder. The nanocontacts became thinner under stretch while the apex angle of the pyramidal shape

kept constant. I found that the volume of the nanocontact was not constant in such deformation

process, indicating and the atoms migrated on the surface around the contact area. The Young’s

modulus, which was calculated from the spring constant and corresponding geometrical information,

was found to decrease when the size was reduced below about 2 nm in diameter. Such size

dependence of the Young’s modulus was analyzed by assuming that the Young’s modulus of the

surface atom layer, the sub-surface atom layer and the core in Au [111] nanocontact were different.

I found that the Young’s modulus of the surface atom layer exhibited very low and one of the core,

the similar value to one of bulk [111] Au crystal. This analysis indicates that the surface atom layer

is softer, which is in consistent of the observed surface atom diffusion. The size dependence of the

Young’s modulus can be explained by surface effect.

Figure 2. (A) histogram of atomic chains with 2-5 atoms, (B) the variation of Young’s modulus with

diameter of the Au nanodisks. The red dots are experiment result and the blue dots is the fitting line

use the core shell model with equation (9), the insert shows the atomic model.

2. Research significance

Nanoscale or atom-scale materials are well known to show various physical or chemical

properties due to quantum confinement effect or surface effect. However, most studies of such

nanoscience have been based on theoretical calculation. Since the physical or chemical properties

in nanoscale are sensitive to the atomic structures, the confirmation of the atomic structure is

required at the same time as measuring the physical or chemical properties. But such method has

been rarely established. In this study, I developed in-situ TEM holder for understanding the elastic

properties of Au and Pt nanocontacts. The originality of our experimental equipment is measuring

the spring constant precisely by using quartz length extension resonator (LER). By LER, I

confirmed that the equivalent spring constant with about 1 N/m in accuracy was obtained based on

frequency modulation method with an extremely small oscillation amplitude of about 30 pm,

indicating that the spring constant of atomic bond is measurable. I believe that our experimental

equipment is the most suitable for estimating the mechanical properties of atom-scale materials in

the world.

The most important results in my study are that the spring constants of gold or platinum

nanowires are obtained precisely. I estimated the individual atom bond stiffness in platinum

monatomic chains, which is obviously higher than the bulk counterpart. Also, I clarified the size

and crystal orientation dependence of Young’s modulus for gold and platinum nanowires or

nanocontacts in this study which has been controversial for a long time and confirmed that Young’s

modulus of gold nanocontacts gradually decreased below 2 nm in diameter due to surface effect.

Such new characterization will open a new field of “atom-mechanics”, leading to atom-scaled

electromechanics and chemicalmechanics.

3. Research Accomplishment

• List of Publications

1. Jiaqi Zhang, Keisuke Ishizuka, Masahiko Tomitori, Toyoko Arai and Yoshifumi Oshima,

“Atomic scale mechanics explored by in situ transmission electron microscopy with a quartz

length-extension resonator as a force sensor” Nanotechnology. 2020, 31, 205706.

2. Jiaqi Zhang, Keisuke Ishizuka, Masahiko Tomitori, Toyoko Arai, Kenta Hongo, Ryo Maezono,

Erio Tosatti, Yoshifumi Oshima, “Atom-mechanics in platinum monatomic chains.” (submitted)

3. Jiaqi Zhang, Keisuke Ishizuka, Masahiko Tomitori, Toyoko Arai, Yoshifumi Oshima, “Size

dependent young’s modulus of sub-2nm gold nanocontacts” (in preparation)

4. Chunmeng Liu , Jiaqi Zhang, Muruganathan Manoharana, Hiroshi Mizuta, Yoshifumi Oshima,

Xiaobin Zhang, “Origin of nonlinear current-voltage curves for suspended zigzag edge

graphene nanoribbon.” Carbon. 2020, 165, 476-483.

5. Chunmeng Liu , Jiaqi Zhang, Xiaobin Zhang, Muruganathan Manoharana, Hiroshi Mizuta,

Yoshifumi Oshima, "In-situ electrical conductance measurement of suspended ultra-narrow

graphene nanoribbons observed via transmission electron microscopy", Nanotechnology 32,

025710 (2020) (8 pages).

• Presentation

1. Jiaqi Zhang, Yuki kobori, Keisuke Ishizuka, Masahiko Tomitori, Toyoko Arai and Yoshifumi

Oshima, "Mechanical Properties of Pt Nano-contacts Measured by TEM Combined with a

Frequency-Modulation Force Sensor", 10th International Conference on Materials for

Advanced Technologies (ICMAT) No. 191177, 25th, June, 2019, Singapore.


Oshima, "Mechanical Properties of Metal Nano-contacts Measured by TEM Combined with

Frequency Modulation Force Sensor" (7P-005), Asian Conference on Nanoscience and

Nanotechnology 2018 October 18-21th, Qingdao (China)


Oshima, “Mechanical properties of Pt nano-contacts measured by TEM combined with a

frequency-modulation force sensing system” The 66th JSAP spring meeting, 9p-PA5-3, 9th,

March, 2019, Tokyo Institute of Technology


Oshima, "Mechanical properties of Pt nano-contacts measured by TEM combined with

frequency modulation force sensor" The 74th Annual Meeting of the Japanese Society of

Microscopy, P-M_48, 8th, June, 2019, Nagoya convention center


"Mechanical properties of Pt atomic chain measured by TEM combined with a frequency-

modulation force sensing system" The 80th JSAP autumn meeting, 19th, September, 2019,

19a-C310-4, Hokkaido University

6. Jiaqi Zhang, Keisuke Ishizuka, Masahiko Tomitori, Toyoko Arai and Yoshifumi Oshima

"Mechanical properties of Pt atomic chain measured by TEM combined with a frequency-

modulation force sensing system" Annual meeting of the Japan Society of Vacuum and Surface

Science 2019, 3Ea05S, 31th, October, 2019, Tsukuba


"Mechanical properties of Pt-Pt bond in an atomic chain measured by TEM combined with a

frequency-modulation force sensing system" The 67th JSAP Spring meeting, 13th, March,

2020, 13p-D519-1~12, Sophia University


"Clarification of Fe ordering in FexTiS2 structure using transmission electron microscopy" The

76th Annual Meeting of the Japanese Society of Microscopy, 2020, 3pmB_S11-6, On-line.


"Atomic scale mechanics measured by TEM holder combined with a frequency-modulation

force sensor", The 81st Autumn Meeting of the Japan Society of Applied Physics, 2020, 10p-

Z06-9 C001981, On-line

• Grant

1. “A study of mechanical properties of Pt atomic contacts by transmission electron microscopy”

Research Report Form for JAIST Research Grant (Fundamental Research) 2019

1,300,000 JPY

2. 「周波数変調型力センサーを組み込んだ TEM観察法による金や白金ナノメカニックスの解明」(2020-2006)、2020 年度笹川科学研究助成、65 万円

("Elucidation of gold and platinum nanomechanics by TEM observation method incorporating

frequency modulation type force sensor" (No.2020-2006), 2020 Sasakawa Scientific Research

Grant, 650,000 JPY)

3. 「原子スケール材料のその場 TEM 計測による弾性特性の研究」、一般財団法人丸文財団第２４回（令和２年度）交流研究助成金、100 万円

("A study of elastic properties of atomic scale materials by in-situ TEM measurement", The

24th Marubun Research Foundation (2020) Exchange Research Grant, 1,000,000JPY)

1

論文題目：繊維状フィラーを利用した高分子複合材料の高機能化

希望取得単位：マテリアルサイエンス

研究室名：山口政之研究室

学生番号： 1820025

氏名：西川理穂

1. 研究内容

本研究では、繊維状フィラーを用いた結晶性高分子の分子配向制御と結晶構造の詳細の解明、

および非相溶ポリマーブレンド中のナノフィラーの分散制御を目的とした。以下に、各章の概要を

簡単にまとめる。

(1) 高分子溶融体中におけるカーボンナノチューブのブラウン運動とネットワーク構造 (第 2章)

高分子溶融体中における多層カーボンナノチューブ (MWCNT) の、ブラウン運動による配向

緩和をレオロジー測定、走査型電子顕微鏡による観察、電気抵抗率の測定により評価した。熱処

理中、ブラウン運動によりMWCNTの粒子間相互作用の寄与が高まり、溶融状態の動的弾性率は

時間とともに成長した。この弾性率成長は、低い温度よりも高い温度において顕著であった。ブラウ

ン運動によって形成された MWCNT のネットワーク構造は、効率的な導電パスである。したがって、

より高い温度で成形したMWCNT含有複合材料は、より高い導電性を示した。

本章の結果は、熱処理温度と時間を制御することにより試料中のナノフィラーのネットワーク構造

や配向のコントロールが可能であることを示す。

(2) カーボンナノチューブからの結晶化を利用した高剛性材料の設計 (第 3章)

MWCNT の添加が流動場において高密度ポリエチレン (HDPE) の分子配向に与える影響、お

よび HDPE の結晶構造の詳細を調べた。HDPE にごく少量の MWCNT を添加し、キャピラリーレ

オメータにて押出成形することで試料を得た。MWCNT 無添加の HDPE ではほとんど配向を示さ

ない条件において、わずか 0.1 wt.%の MWCNT 添加で HDPE 鎖は分子配向 (シシカバブ構造)

を示すことが明らかになった。これは、流動方向に配向した MWCNT が HDPE の結晶核剤として

働くことにより、冷却過程で MWCNT が HDPE 鎖のシシとして作用し、結晶化速度を速めたことに

起因する。さらに、配向度の向上に伴い引張弾性率も飛躍的に増加した。

本成果は、ごく少量の繊維状結晶核剤を添加することで、汎用的な成形条件にて高分子鎖の

分子配向を大きく向上できることを示唆している。

(3) 共連続ポリマーブレンド中におけるカーボンナノチューブの界面局在化 (第 4章)

ポリカーボネート (PC) /ポリエチレン (PE) 共連続相の界面にMWCNTを局在化させることで、

2

ポリマーナノコンポジットに良好な導電性を与える手法を提案した。PE 成分として超高分子量 PE

(UHMWPE) を用いることで PE相の粘度を高く設定したところ、MWCNTは PC相とUHMWPE相

の界面に局在化した。これは、当初 PC 相に存在した MWCNT が混練中に UHMWPE 相へと移

行しようとするが、MWCNTの低い拡散係数のため UHMWPE相中には入り込めなかったことに起

因する。さらに、本ブレンドは共連続構造を形成するため、MWCNT は効率的な導電パスとして機

能し、試料は良好な導電性を示した。

この章では、濡れ係数を用いた予測では不可能であったフィラーの界面局在を実現した。した

がって、これらの成果は、ナノフィラーを用いた導電性ポリマーブレンドの新たな知見であり、材料

設計の幅を広げると考えられる。

(4) 高分子溶融体中における繊維形成を利用したポリビニルアルコール繊維強化プラスチックの

設計 (第 5章)

溶融ポリプロピレン (PP) 中におけるポリビニルアルコール (PVA) の繊維化を達成するために、

以下の二つの方法を試みた。一つ目は、PP と低分子量の PVA を溶融混練した後、溶融延伸を行

う方法である。二つ目は、PVA 水溶液を溶融 PP に添加し脱気を行いながら二軸押出機にて混練

する方法である。両方法ともに、繊維状の PVA を得ることができ、さらに、PVA は PPに対して結晶

核剤能力を示すことが判明した。続いて行われた射出成形過程において、配向した PVA 繊維に

沿って PP鎖が結晶化することにより、成形後の試験片は高い分子配向度と力学特性を示した。

PVA 繊維はガラス繊維よりも軽量であり、さらに近年着目されているカーボンナノチューブやセ

ルロースナノファイバーよりも安価である。したがって、本手法は自動車部品などへの応用が期待さ

れる。さらに、本技術は他の高分子へも応用可能と考えられるため、汎用性が高い。

2. 研究の意義

プラスチック成形加工は、そのほとんど全てにおいて溶融時の流動場を利用して行われている。

高分子複合体における流動条件はフィラーの分散状態に影響し、さらに繊維状フィラーを用いた

際はその配向の異方性にも影響を及ぼす。これらは成形後の性能や物性に大きく関係する。近年、

ナノスケールのフィラーの複合化研究が盛んであるにもかかわらず、高分子溶融体中におけるナノ

フィラーの挙動を詳細に研究した例はほとんどない。本研究では、熱処理温度やマトリクスポリマー

の粘度に着目し、MWCNT のブラウン運動を制御することで効率的にネットワーク構造やパーコレ

ーションを得る方法を提案した。繊維状ナノフィラーの分散状態および配向は、流動場による影響

とブラウン運動による拡散の競合によって決定されるためである。また、ナノフィラーは、高分子鎖

のサイズに近いためマトリクスポリマーの構造に影響を与えやすい。しかし、複合材料中における

マトリクスの結晶構造の詳細など未だ不明な点が多い。本研究では、MWCNT を結晶核 (伸長鎖

3

結晶／シシ) とした HDPE の構造を詳細に調査した。これらの成果は、ナノコンポジット設計の有

用な知見となり得る。

また、工業的な応用には、軽量かつ原料が汎用性に優れた材料が必要とされる。そのような繊

維として、例えば PVA繊維が挙げられる。しかし、PVAのような水溶性高分子の、汎用高分子中に

おける均一分散は困難とされてきた。本研究では、分散相である PVA の粘度に着目することによ

り、溶融マトリクス中における PVAの繊維化および均一分散を実現した。

本研究で得られた成果が、次世代の材料設計や成形加工技術の向上に役立つこととなれば幸

いである。

3. 研究業績

以下に研究業績を記す。なお、学会発表に関しては主要なものを抜粋して記載する。

原著論文 (査読有り)

1. Nishikawa, R.; Yoon, H.; Yamaguchi, M.

“Rheological evaluation of carbon nanotube redistribution in polymer melt”

Journal of Society of Rheology Japan (Nihon Reoroji Gakkaishi) 2019, 47 (3), 105–110.

2. Nishikawa, R.; Yamaguchi, M.

“Effect of carbon nanotube addition on structure and properties for extrudates of high-density

polyethylene”

Journal of Applied Polymer Science 2019, 136 (40), 48010.

3. Nishikawa, R.; Tamaki, K.; Notoya, O.; Yamaguchi, M.

“Carbon nanotube localization at interface in co-continuous blends of polyethylene and

polycarbonate”

Journal of Applied Polymer Science 2019, 137 (10), 48676.

4. Nishikawa, R.; Aridome, N.; Ojima, N.; Yamaguchi, M.

“Structure and properties of fiber-reinforced polypropylene prepared by direct incorporation of

aqueous solution of poly(vinyl alcohol)”

Polymer 2019, 199, 122566.

学会発表

(国際学会ポスター発表、査読有り)


“Effect of CNT addition on structure of HDPE”

Asian Workshop on Polymer Processing 2018, Asian Workshop on Polymer Processing 2018,

4

Chiang Mai, Thailand, December 2018.


“Impact of carbon nanotube addition on structure of high-density polyethylene”

Annual Technical Conference of Society Plastics Engineers 2019, Detroit, America, March 2019.


“Structure change of high-density polyethylene by addition of carbon nanotube”

European Polymer Congress 2019 Crete, Greece, June 2019.

(国内学会口頭発表、査読無し)

8. 西川理穂; 山口政之

カーボンナノチューブ添加による高密度ポリエチレンの配向状態制御

第 64回レオロジー討論会, 彦根, 2019年 10月

9. 西川理穂; Panitha Phulkerd; 山口政之; 小島直紀; 有留憲文

ポリビニルアルコール水溶液添加による高性能ポリプロピレンの設計

プラスチック成形加工学会第 27回秋季大会, 高松, 2019年 11月

10. 西川理穂; 山口政之; 小島直紀; 有留憲文

ポリビニルアルコール水溶液の添加により調製したポリプロピレン系コンポジットの構造と物性

第 69回高分子討論会, オンライン, 2020月 9月

11. 西川理穂; 山口政之; 小島直紀; 有留憲文

水溶液添加法により調製したポリプロピレン/ポリビニルアルコールの構造と物性

第 65回レオロジー討論会, オンライン, 2020年 10月


カーボンナノチューブを利用したポリエチレンの配向制御

プラスチック成形加工学会第 28回秋季大会, オンライン, 2020年 12月

(国内学会ポスター発表、査読無し)


ポリマー溶融体中におけるカーボンナノチューブのネットワーク形成

プラスチック成形加工学会第 26回秋季大会, 浜松, 2018年 11月


カーボンナノチューブ添加による高密度ポリエチレンの構造変化

第 68回高分子学会年次大会, 大阪, 2019年 5月

受賞等

15. 日本学術振興会特別研究員 DC2 採用 (令和 2年度)

Ab initio assessment of computational thermodynamics applied to magnetic alloys

School of Materials Science Maezono Laboratory

Adie Tri Hanindriyo 1820027

Research Content

Permanent magnets are widely used in society, from small-scale, everyday applications (fridge magnets, paper holders) to large-scale, industrial needs (electric generators, motors). The rare earth compound Nd2Fe14B is well-known as the base material for the strongest permanent magnets available today, colloquially known as “neodymium magnets”.1 This is due to the high level of magnetization (strength of magnetic field) and coercivity (resistance to demagnetization by external field) exhibited by this ternary rare earth compound.

Even so, improvements upon the base material are often desired to fit a variety of applications. One famous example is the effort to improve upon the Curie temperature (Tc), a thermodynamic property describing the temperature at which magnetic materials lose their magnetic ordering. Increasing Tc is desirable for applications in which permanent magnets face a high working temperature, which can compromise magnetic field strength. An example of this effort is the alloying of Nd2Fe14B with cobalt (Co), which substitutes several Fe sites with Co (Nd2Fe14-xCoxB). 2 Further alloying with dysprosium (Dy), substituting Nd sites, is made to make up for some coercivity lost upon alloying with Co.3

However, due to dependence on China for global Dy production, it is desirable to find alternative ways to improve upon the Nd2Fe14B compound.

Atomic substitution and alloying for property control requires proper information on phase

1 Sagawa, et al. Jpn. H. Appl. Phys. 26 785 (1987) 2 Bolzoni, et al. J. Appl. Phys. 61 5369 (1987) 3 Chen, et al. J. Magn. Magn. Mater. 261 222 (2003)

Figure 1 Crystal structures of Nd-Fe-B ternary system constituent compounds investigated in this work, (a) NdB6, (b) NdB4, (c) Nd2B5, (d) Nd2Fe17, and (e) Nd5Fe2B6

stability, which is embodied within the phase diagram. Phase diagrams show the conditions (temperature, pressure, etc.) on which certain phases can be stably realized. The computational thermodynamics method CALPHAD (CALculation of PHAse Diagrams) is often employed to collate meaningful experimental information on constituent phases and compile the information to draw the phase diagram. The core of CALPHAD is the mathematical modelling of Gibbs energy of constituent phases in a system, from which reliable predictions of phase transitions can be made (leading to the complete phase diagram). In order to reliably model the Gibbs energy, accurate information of thermodynamic properties is in turn

required, which is why information on thermodynamic properties of constituent phases is highly valuable for CALPHAD researchers.

Several CALPHAD assessments of the Nd-Fe-B ternary system have been performed,456789 but the lack of experimental information on thermodynamic properties of Nd-Fe-B constituent phases have limited the reliability of the Gibbs energy models used within these assessments. While the constituent Fe-B binary system is relatively well-documented (due to its role in hardened steel alloys), the Nd-B, Nd-Fe, and Nd-Fe-B systems contain phases for which experimental data is not readily available. In order to compensate for this fact, this work attempts to use ab initio methods to calculate and predict relevant thermodynamic information of the constituent phases of the Nd-B, Nd-Fe, and Nd-Fe-B systems (shown in Figure 1) for use as input data for CALPHAD.

Two primary thermodynamic properties, enthalpy of formation (ΔHf) and specific heat in constant pressure (Cp), corresponding to the enthalpy and entropy terms of the Gibbs energy model, respectively, are calculated using the ab initio Density Functional Theory (DFT) with the Generalized Gradient Approximation (GGA) as a conventional choice of exchange-correlation functional. It is observed that, likely due to the incomplete self-interaction error well-known to occur with conventional exchange-correlation functionals and localized electron orbitals (3d/4f

4 Matsuura, et al. Jpn. J. Appl. Phys. 24 L635 (1985) 5 Hallemans, Wollants, and Roos. J. Phase Equilibria 16(2) 137 (1995) 6 Raghavan. J. Phase Equilibria 24(5) 451 (2003) 7 Van Ende and Jung. J. Alloys Compd. 548 133 (2013) 8 Zhou, Luo, and Zhou. J. Electron. Mater. 45(1) 418 (2016) 9 Chen, et al. Calphad 66 101627 (2019)

Nd2 B

5

NdB4

NdB6

NdB66

1127.8

1279.0

2272.9

2880.6

2673.6

1388.0

2395.1

2348.0

T[K]

500

1000

1500

2000

2500

3000

X(Nd)

0 0.2 0.4 0.6 0.8 1

Figure 2 Calculated phase diagram of the binary Nd-B system in this work

orbitals in transition metals and rare earth compounds), the enthalpy of formation calculated for several phases possess significant discrepancy with experimental results. To address this issue, the Hubbard U correction according to the ab initio framework of Cococcioni and de Gironcoli10 was employed, which reduced the discrepancy to a tolerable level.

Phonon calculations using the finite difference frozen phonon approximation was used to calculate phonon frequencies of phases in the harmonic approximation to calculate specific heat in constant volume (Cv). Subsequently, the quasi-harmonic approximation was employed to introduce volume dependence and calculate Cp. The enthalpy of formation and specific heat information was then used in subsequent CALPHAD method to reassess the phase diagram of the binary Nd-B system, one of the important constituent systems of the ternary Nd-Fe-B system. Figure 2 shows the end result of CALPHAD reoptimization described in this work for the binary Nd-B system.

Research Purpose

The DFT-CALPHAD framework employed in this work is a recent possibility explored to complement experimental and empirical data, with several important advantages. One of the most important of which is the investigation of microstructures and phases which might be very costly (in both time and energy) to synthesize, when ab initio methods can reliably provide an accurate prediction at a fraction of the cost. This is the first such work to apply the DFT-CALPHAD framework, and also the DFT+U technique within the same framework, to the Nd-Fe-B system, and the intriguing results discovered suggest that this will also not be the last application of this framework for this system, as it is a flexible framework which can be used to investigate quarternary and quinary systems as well (for example, the Nd-Fe-Co-B quarternary and Nd-Dy-Fe-Co-B quinary systems relevant for Nd2Fe14-xCoxB phase and (Nd,Dy)(Fe,Co)B magnet materials, respectively).

The resulting binary Nd-B phase diagram is a more reliable prediction of phase stability as a result of incorporating ab initio data to the Nd-Fe-B database. Significant improvement was made specifically toward the Nd-rich region of the composition space, particularly relating to the liquidus line which separates the liquid phase from a multitude of stable solid phases. As a particularly model-dependent area of the phase diagram, the liquidus line of the Nd-B phase diagram has seen substantial changes across multiple reassessments relying on a relatively small amount of experimental thermodynamic data. By compensating for this lack of input information with ab initio assessments of the Nd-B constituent phases, this work has more reliably predicted the liquidus line of the Nd-B phase diagram.

In particular to the Nd2B5 system, this work has also shown that the discrepancy of ab initio DFT calculations with respect to experimental data (as found in preceding work of Colinet and Tedenac11) is not improved by the use of the Hubbard U correction, and is likely caused by more than the suspected self-interaction error. As this discrepancy is also noted for the isomorphic Gd2B5 compound, it is likely that an aspect of the crystal structure is improperly described by the GGA exchange correlation functional.

10 Cococcioni and de Gironcoli. Phys. Rev. B 71 035105 (2005) 11 Colinet and Tedenac. Calphad 62 49 (2018)

Research Accomplishment

Peer reviewed

[1] A. T. Hanindriyo, TB. M. Y. Y. Prawira, M. K. Agusta, and H. K. Dipojono. “Computational Design of Ni-Zn Based Catalyst for Direct Hydrazine Fuel Cell Catalyst using Density Functional Theory”. Procedia Engineering 170 148-153 (2017)

[2] A. T. Hanindriyo, S. Sridar, K. C. Hari Kumar, K. Hongo, and R. Maezono. “Ab initio thermodynamic properties of certain compounds in Nd-Fe-B system”. Computational Material Science 180 109696 (2020)

[3] A. Hermawan, A. T. Hanindriyo, E. R. Ramadhan, Y. Asakura, T. Hasegawa, K. Hongo, M. Inada, R. Maezono, and S. Yin. “Octahedral morphology of NiO with (111) facet synthesized from the transformation of NiOHCl for the NOx detection and degradation: experiment and DFT calculation”. Inorganic Chemistry Frontiers 7 3431-3442 (2020)

[4] T. Hasegawa, A. Shigee, Y. Nishiwaki, M. Nagasako, A. T. Hanindriyo, K. Hongo, R. Maezono, T. Ueda, and S. Yin. “New layered perovskite family built from [CeTa2O7]- layers: coloring mechanism from unique multi-transitions”. Chemical Communications 56 8591-8594 (2020)

[5] A. Raghav, A. T. Hanindriyo, K. Utimula, M. Abbasnejad, R. Maezono, and E. Panda. “Intrinsic electronic defect states of anatase using density functional theory”. Computational Materials Science 184 109925 (2020)

Non peer reviewed

[1] A. T. Hanindriyo, A. K. S. Yadav, T. Ichibha, R. Maezono, K. Nakano, and K. Hongo. “Diffusion Monte Carlo evaluation of disiloxane linearization barrier”, arXiV:2010.06767 [physics.chem-ph] (2020)

Studies on CuPd Bimetallic Catalysts forSelective Hydrogenation of Succinic Acid

Doctoral DegreeNishimura’s LaboratoryLe Dinh Son (1820036)

1 Research Content

1.1 Research background and objective

Succinic acid (SA) was identified as one of the most potentially bio-derivedplatform chemicals which can be converted to various value-added products.Hydrogenation is by far the most investigated transformation due to the impor-tance of its products including γ-butyrolactone (GBL), tetrahydrofuran (THF),and 1,4-butanediol (BDO). However, the selective hydrogenation of SA is gener-ally a challenging reaction due to the low electrophilicity of the carbonyl groupand the complexity in its reaction pathways, which have provided a strong spurfor chemists to design effective catalysts for this transformation. Despite that,the heavy dependence on precious metals such as Pd, Pt, Re, Ir, Ru, and Rhin previously reported catalysts is economically disadvantageous, which possi-bly limits them from industrial applications. Therefore, the studies embodiedin this thesis aim to develop efficiently earth-abundant metal-based bimetalliccatalysts for selective hydrogenation of SA to GBL, BDO, and THF (Figure 1).

Figure 1: Selective hydrogenation of SA to GBL, BDO, and THF

1.2 Results and discussion

In the initial attempt to search for a suitable catalyst system, hydroxyapatite(HAP) supported CuxPdy (x+y = 10 wt%) was found to be potential bimetalliccatalysts for the production of BDO from SA. The effect of metal ratio was ex-amined and the Cu8Pd2/HAP was found to be the best catalyst, affording a highselectivity of BDO (>80%) at a quantitative conversion of SA. A strong Cu–Pdinteraction resulted from alloying formation led to an enhanced catalytic activityto the intermediate GBL, compared to that over the Cu10/HAP monometalliccatalyst. While on the other hand, the Cu-rich CuPd nanoparticles (NPs) sup-pressed the over-reactivity of Pd, preventing the side reaction to butyric acid(BA), which is typically encountered in the Pd10/HAP monometallic catalyst.Subsequently, the Cu species that existed adjacently to CuPd alloying NPs pro-moted further hydrogenation of GBL toward BDO.

1

The influences of various supports, i.e., SiO2, TiO2, and γ-Al2O3, on theconstructions of Cu-rich CuPd alloy nanoparticles (NPs) were then investi-gated. In-depth characterizations revealed that randomly homogeneous CuPdNPs were prevalently constructed on TiO2 and SiO2, whereas the heterogeneousCuPd alloy NPs with a great extent of Cu segregation were dominantly formedon γ-Al2O3. Accordingly, the activity and selectivity are distinctly differentamong these catalysts. Particularly, a selectivity of GBL (90%) can be attainedover the CuPd/TiO2 at 73% conversion of SA, which was attributed to the pres-ence of large CuPd NPs preventing further hydrogenation of GBL and loweringthe catalytic activity. On the other hand, higher activity and selectivity towardBDO of CuPd/SiO2 were ascribed to its small CuPd NPs and the presence ofisolated Cu species which promoted the formation of BDO at a high yield of86%. Notably, the strong Lewis acid sites in the CuPd/γ-Al2O3 was revealedas the decisive factor in the formation of highly selective THF with 97% at aquantitative conversion of SA.

To broaden knowledge in the γ-Al2O3 supported CuPd catalysts, the influ-ence metal ratio on the catalytic performance has been extended. Excellent cat-alytic performance toward THF was achieved over the Cu-rich Cu6Pd4/γ-Al2O3

and Cu8Pd2/γ-Al2O3 catalysts, achieving the product yield and selectivity of85–90%. In addition, the present catalyst can maintain its high activity andselectivity for several recycling runs under high temperature and pressure con-ditions. The strong interaction in CuPd alloy NPs resulted in the enhancedreactivity compared to that of the monometallic Cu, while the Cu-rich com-ponent helped to restrain the strong reactivity of Pd species which favors theformation of BA. Subsequently, the Cu-rich CuPd NPs promoted the formationof the intermediate BDO which was easily converted to THF via cyclodehydra-tion under the influence of strong Lewis acid sites in the support γ-Al2O3.

Finally, the influence of the capping agent on the catalytic performance ofCuPd NPs was studied. A highly efficient PVP-capped CuPd NPs constructedon HAP was discovered for selective hydrogenation of SA to GBL. The inhi-bition effect of the capping agent PVP was revealed to play a key role in theformation of GBL with excellent selectivity. The catalyst was able to proceed atextremely low hydrogen pressure from 1 MPa while maintaining high selectivityof GBL (>90%). Besides, the catalyst showed remarkable reusability, offeringthe catalyst with enormous potential for industrial applications.

2 Key findings and contributions

2.1 Tunable catalysts for selective SA hydrogenation

The importance of SA hydrogenation which generates the key intermediatesincluding GBL, BDO, and THF for the polymer industry is unquestionable.The discovery of CuPd bimetallic catalysts in the present research, therefore,provides not only cheaper catalysts but also effective and feasible methods totune the product selectivity for the SA hydrogenation (Table 1). Furthermore,in terms of productivity, the present catalyst system showed excellent perfor-mances, which closely approach or even greatly superior to the highest recordsfor BDO or THF and GBL productions so far. On the other hand, in the con-text of reaction conditions, the HAP supported PVP-capped Cu40Pd60 catalyst

2

promoted the SA hydrogenation to highly selective GBL even at low hydro-gen pressure, from 1 MPa. In addition, the extremely low concentration ofmetal loading (0.1 mmol) in combination with the excellent reusability madethis catalyst suitable for flow reactor and thus open up a possibility for indus-trial applications.

Table 1: Catalytic hydrogenation of SA

CatalystTemp.

/℃P H2

/MPaConv.

/%Selectivity /%

BDO GBL THFCu8 –Pd2/HAP or SiO2 200 8 100 82–86 <5 ≤3Cu8 –Pd2/γ-Al2O3 200 8 100 0 0 97Cu8 –Pd2/rutile-TiO2 200 8 73 2 90 3Cu40 –Pd60 –PVP/HAP 200 1–4 68–92 0 92–96 0Ir–Re/C 240 8 100 <5 0 75Pd–Cu/AX 190 7 72 0 94 0Pd/Al2O3 170 3 <70 <1 95 0Re–Ru/C 160 8 99 70 6 6Re3–Ru/C 240 8 99 3 5 60Pd–Re/AC 180 10 100 67 0 14Re–Ru/BMC 200 8 100 65 34 3Re–Pd/SiO2 140 8 100 89 3 0Pd–5 FeOx/C 200 5 100 70 - -Pd–Re/C 240 8 89 4 - 73Pd–Re/TiO2 160 15 100 83 0 -

The red color texts indicate the present study, while the remaining werereported in the open literature in the last decade.

2.2 Mechanism of SA hydrogenation

(1) SA → GBL: According to the catalytic performances of monometalliccatalysts, both Cu and Pd can reduce SA to GBL. However, Cu catalysts showedpoor catalytic performance for SA hydrogenation which can be attributed tothe limited capability of Cu for H2 activation. On the other hand, Pd holds agreat potential for H2 dissociation, which turned out not to be suitable for SAhydrogenation since it directed the reaction to the side product BA. The strongCu–Pd alloying formation resulted in better GBL yield in the Cu8Pd2/TiO2

and Cu40Pd60 –PVP/HAP catalysts. The reactions were terminated at GBLwithout further hydrogenation because the absence of isolated Cu and/or largeCuPd NPs of the former catalyst and the inhibition effect of PVP in the latter.

(2) SA → BDO: The reaction using GBL as a starting material showedthat Cu can reduce GBL to afford BDO with excellent yield and selectivity.Therefore, the presence of isolated Cu in Cu8Pd2/HAP and Cu8Pd2/SiO2 canfurther reduce the intermediate GBL to BDO with high yield and selectivity. Itshould be emphasized that the close interaction between Cu and Pd is impor-tant because the physical mixture of Cu8/HAP and Pd2/HAP were unable toproduce BDO with the comparable yield to that over the bimetallic catalyst.

3

(3) SA → THF: The reaction using BDO as starting material revealedthat strong Lewis acid sites in γ-Al2O3 is one of the crucial factors for theproduction of THF. However, CuPd alloy is the prerequisite factor for directhydrogenation of SA to THF since the reaction over the bare support was failedto produce any THF. The hydrogenation of GBL over Cu8Pd2/γ-Al2O3 indi-cated that in the preliminary step of this reaction, BDO was formed as anintermediate, which immediately converted to THF under the influence of thestrong Lewis acid site. Therefore, the highly selective of THF via SA hydro-genation requires the combination and working in concert of the Cu-rich CuPdNPs and the strong Lewis acid sites of γ-Al2O3.

2.3 Conclusion

The present thesis provides feasible and versatile methods to design effectivecatalysts for selective hydrogenation of SA. The SA hydrogenation could be di-rected toward a specific product including GBL, BDO, and THF by adjustingthe metal ratio, changing the catalyst support, and stabilizing with cappingagent. The roles of metals, support, and capping agent have been revealed byextensive characterizations. In addition, the reaction pathways and optimizedreaction conditions for each integrated reaction have been clarified and exam-ined. The mentioned understanding of the current catalyst system allows us tocontrol the SA hydrogenation and other related reactions from various angles.

3 Research Accomplishment

Academic journals – peer-reviewed

1. Son Dinh Le, Shun Nishimura, Hydroxyapatite Supported Polyvinylpyrro-lidone-Capped CuPd Nanoparticles for Highly Selective Lactonization ofSuccinic Acid. (In preparation)

2. Son Dinh Le, Shun Nishimura, Influence of Metal Ratio on Alumina-Supported CuPd Catalysts for the Production of Tetrahydrofuran fromSuccinic Acid. (Submitted, under review)

3. Son Dinh Le, Shun Nishimura, Effect of support on the formation of CuPdalloy nanoparticles for the hydrogenation of succinic acid, Appl. Catal. B:Environ., 2021, 282, 119619.

4. Son Dinh Le, Shun Nishimura, Highly selective synthesis of 1,4-butanediolvia hydrogenation of succinic acid with supported Cu–Pd alloy nanopar-ticles, ACS Sustain. Chem. Eng., 2019, 7, 18483–18492.

Patent applications

5. Shun Nishimura, Son Dinh Le, Catalysts for 1,4-butanediol, butyric acidand tetrahydrofuran productions, Japanese Patent Application Nos. 2020-115838 (filed July 3, 2020) and 2019-148506 (filed August 13, 2019).

4

1

Studies on Degradation Mechanism of Fluorescent Organic Light Emitting Diodes Based on

Triplet-Triplet Annihilation by Time-resolved Luminescent Spectroscopies

(Materials Science)

Murata laboratory

ID: 1720433

Name: LE Cong Duy

Organic light emitting-diodes (OLEDs) are an important technology for high-quality flat panel displays as

well as lighting owing to their self-emission, high contrast ratio, wide viewing angle, and fast response.

New attractive applications are enabled by OLEDs, such as flexible,1 wearable,2 and transparent diplays.3

Blue fluorescent OLEDs that utilize triplet-triplet annihilation (TTA) are employed for blue pixels of small

OLED displays used for smart phones.4 TTA based blue OLEDs, however, demonstrates inferior operation

lifetime compared with those of green and red phosphorescent OLEDs and requires further improvement.4

To overcome the short operation lifetime, understanding degradation mechanisms of TTA based OLEDs

are essential. Emission of TTA based OLEDs is composed of prompt electroluminescence (EL) from singlet

exciton that is directly generated by charge recombination and delayed EL from additional singlet exciton

generated via TTA process. Hence, degradation mechanism of the OLEDs would be ascribed to the

reduction of the density of singlet exciton during the device operation.

In this research, we aim at investigating the degradation mechanisms of TTA based fluorescent OLEDs

with time-resolved luminescent spectroscopies. Degradation owing to the decrease in prompt EL was

investigated by time-resolved photoluminescence (TRPL) spectroscopy. In addition, degradation ascribed

to the decrease in delayed EL was investigated by time-resolved electroluminescence (TREL) spectroscopy.

Degradation of green fluorescent OLEDs

Figure 1 demonstrates EL performance of pristine and

degraded green OLEDs using tris(8-hydroxyquinolinolato)

aluminum (III) (Alq3) as an emitter. Emission of Alq3-based

OLED uses both singlet exciton formed by charge

recombination and TTA.5 External quantum efficiency (EQE)

at 50 mA/cm2 of the degraded device shows 1.04%, which is

90% of pristine one (1.15%).

The device degradation was investigated in terms of

parameters of the EQE expressed as follows

𝐸𝑄𝐸 = 𝛾𝜂𝑜𝑢𝑡𝜙𝑃𝐿(𝜂𝑆 + 𝜂𝑇𝑇𝐴), (1)

where is charge balance factor, out is out-coupling

efficiency, PL is PL quantum yield, S and TTA are fractions

of singlet exciton formed by charge recombination and by

TTA, respectively. Assuming and out are constant as devices

age, the decrease in the EQE is attributed to the decrease in PL

and/or (S + TTA).

To investigate change in PL after the degradation, we have

measured TRPL of pristine and degraded devices. Singlet

lifetime (τ) of Alq3 was obtained from fitting TRPL curves

(Fig. 2). Ratio of PL(0.9) over PL(1.0) corresponds to ratio of

τ(0.9) (13.5 ns) over τ(1.0) (14.3 ns) that is 94%. This indicates

that device degradation is partly ascribed to a reduction in PL

owing to singlet quenching with neutral quenchers (Fig. 4).

Figure 1. External quantum efficiency (EQE),

EL spectrum (in set) of pristine (1.0) and

degraded (0.9) devices.

Figure 2. Transient PL of pristine (1.0) and

degraded (0.9) devices. Fitting equation:

IPL (t)=exp(-t/τ)

2

Neutral quenchers may act as hole traps during device

operation, which is confirmed by capacitance-voltage

measurement of the degraded device. To examine the effect of

the trapped positive charges on the device degradation, we

performed TREL of the devices, the pristine device shows that

delayed EL that is generated via TTA process decays within 16

s (Fig. 3). A reduction in delayed EL intensity of the degraded

device suggests that the additional 4 points out of 10%

degradation could be ascribed to the reduction in TTA, which is

attributed to quenching of triplet excitons with trapped positive

charges (Fig. 4).

Hence, the initial 10% luminance loss of green fluorescent

OLEDs is ascribed to the decrease of PL (6 points) due to

quenching of singlet exciton with neutral quenchers and the

decrease of the generation of additional singlet exciton via the

TTA process (4 points) owing to quenching of triplet excitons

with trapped positive quenchers (Fig. 4).

Degradation of highly efficient blue fluorescent OLEDs

Blue OLED where the emitting layer (EML) was composed of

a host material and emitting dopant demonstrates a high EQE

of 9.56% (Fig. 5), which exceeds the theoretical limit (5.0%) of

fluorescence OLEDs that make use of only singlet exciton

generated by charge recombination. A similar decay time of

delayed EL between OLEDs with and without emitting dopant

suggests that the blue OLED also utilizes the delayed EL that is

generated via TTA taking place in the host material (Fig. 6).

The luminance OLED with emitting dopant reaches 95% of the

initial 1,650 cd/m2 luminance after 732 h continuous operation

(LT95), as shown in Table 1. Importantly, the OLED

demonstrates better stability even at higher brightness operation

compared to the state-of-the-art data.6 For degradation analysis,

the OLED was further degraded until luminance reaches 87%

of the initial luminance.

To investigate change in PL after the degradation, we have

measured TRPL of pristine and degraded devices. No change

in singlet lifetime of dopant for pristine and degraded OLEDs

suggests that device degradation is not ascribed to a decrease in

PL of the dopant owing to quenching of singlet exciton with

neutral quenchers.

To investigate change in TTA after the degradation, we have

measured TREL of pristine and degraded devices. A small

reduction in delayed EL intensity of a degraded OLED with

emitting dopant suggests that the device degradation is ascribed

to the decrease in TTA (Fig. 7a). Moreover, a similar reduction

was also observed for a degraded OLED without emitting

dopant confirms that the decrease TTA is due to the decrease in

Figure 5. External quantum efficiency (EQE)

and EL spectrum (in set) of a blue OLED.

Figure 3. Transient EL at 50 mA/cm2 of

pristine (1.0) and degraded (0.9) devices.

Figure 4. Degradation mechanisms of green

OLEDs.


OLEDs with and without emitting dopant.

measured at 15 mA/cm2. Table 1. Summary of EL performance of device

3

density of singlet exciton generated via the TTA of the host

material (Fig. 7b). The origin of the decrease in singlet exciton

density will be discussed in the concept of quenching of triplet

excitons with positive quenchers.

Hence, the blue OLED with high efficiency and excellent

operation lifetime is realized. The device degradation is

ascribed to the decrease in the delay EL generated via TTA of

the host material.

Research Originality: Transient luminescent spectroscopies

were employed to investigate degradation mechanisms of TTA-

based green and blue OLEDs.

Research Novelty:

For degradation of green fluorescent OLEDs, initial 10%

luminance loss of devices is ascribed to the decrease in PL

quantum yield (6 points) owning to singlet quenching with

neutral quenchers and the decrease in additional singlet exciton

generated via TTA (4 points) due to triplet quenching with

positively trapped quenchers.

For degradation of highly efficient blue fluorescent OLEDs,

device degradation is ascribed to a decrease in additional singlet

exciton generated via the TTA of the host material.

Research Achievement

- Publications

1. Fivefold enhancement in stability of organic light emitting diodes with addition of non-evaporable getter

pumps, D. C. Le, S. Oyama, H. Sakai, T. Porcelli, F. Siviero, E. Maccallini, M. Urbano and H. Murata,

Applied Physics Express, with peer review, 10, 071601(1−4), 2017.

2. Investigation of the improvement in short-term stability by post annealing of fluorescent organic light

emitting diodes, D. C. Le and H. Murata, Int. SID Symposium Digest of Technical Papers, without peer

review, 51 (1), 2027-2029, 2020.

3. Degradation of fluorescent organic light emitting diodes caused by quenching of singlet and triplet

excitons, D. C. Le, D. D. Nguyen, S. R. Lloyd, T. Suzuki and H. Murata, Journal of Materials Chemistry

C, with peer review, 8, 14873-14879, 2020.

- International Conferences

1. Effect of post-treatment on the stability of organic light emitting diodes, D. C. Le, H. Sakai and H.

Murata, the 19th international workshop on inorganic and organic electroluminescence and 2018

international conference on the science and technology of emissive displays and lightning, Tokyo, Japan,

September 11−13, 2018.

2. Investigation of degradation mechanisms of highly efficient blue fluorescent organic light emitting

diodes through analysis of exciton dynamics, D. C. Le, A. Asagatani, H, Kubota, H. Sakai and H.

Murata, the 10th international conference on molecular electronics and bioelectronics, Nara, Japan, June

25−27, 2019.

3. Analysis of efficiency roll-off in blue fluorescent organic light emitting diodes utilizing triplet-triplet


pristine (1.0) and degraded (0.87) devices

(a) with and (b) without emitting dopant.

measured at 15 mA/cm2.

4

annihilation by transient El, S. Asagatani, D. C. Le, H. Kubota, H. Sakai and H. Murata, the 10th

international conference on molecular electronics and bioelectronics, Nara, Japan, June 25−27, 2019.

4. Investigation of the reliability enhancement by annealing of organic light emitting diodes fabricated in

a chamber with extremely low concentration of residual water, D. C. Le, L. Yining, H. Sakai and H.

Murata, SPIE Optics and Photonics, San Diego, USA, August 11−15, 2019.

5. Investigation of the improvement in short-term stability by post annealing of fluorescent organic light

emitting diodes with analysis of exciton dynamics, D. C. Le and H. Murata, SID Display week

symposium, online, August 3−7, 2020.

6. Extended lifetime of fluorescent organic light emitting diodes by combining ultra-clean fabrication

conditions and annealing after device fabrication, D. C. Le and H. Murata, the 12th Asian conference

on organic electronics, online, November 8−10, 2020.

7. Mechanisms of initial luminance loss of fluorescent organic light emitting diodes unveiled by time-

resolved spectroscopies, D. C. Le, D. D. Nguyen, S. R. Lloyd, T. Suzuki and H. Murata, the 27th

international display workshops, online, December 9−11, 2020.

- Domestic Conferences

1. Effect of experimental data length on luminance decay analysis using composite exponential function,

R. Matsumoto, D. C. Le, H. Sakai and H. Murata, 65th spring meeting of Japan society of applied

physics, Tokyo, Japan, March 17−20, 2018.

2. Twofold enhancement in reliability of organic light emitting diodes with thermally induced

morphological change of organic layer, D. C. Le, L. Yining, H. Sakai and H. Murata, 66th spring

meeting of Japan society of applied physics, Tokyo, Japan, March 9−12, 2019.

3. Roll-off analysis of TTA type blue light emitting organic EL element using time-resolved EL, S.

Asagatani, D. C. Le, H. Kubota, H. Sakai and H. Murata, 28th meeting of the Japan OLED forum,

Tokyo, Japan, June 13−14, 2019.

4. Exciton dynamics analysis in organic light emitting diodes under operation environment, S. Asagatani,

D. C. Le and H. Murata, 67th spring meeting of Japan society of applied physics, Tokyo, Japan, March

12−15, 2020.

- Best Poster Presentation Awards

1. Significant lifetime enhancement of organic light emitting diodes by removing residual water during

device fabrication, D. C. Le, S. Oyama, H. Sakai and H. Murata, 9th international conference on

molecular electronics and bioelectronics, Kanazawa, Japan, June 26−28, 2017.

2. Extended lifetime of fluorescent organic light emitting diodes by combining ultra-clean fabrication

conditions and annealing after device fabrication, D. C. Le and H. Murata, the 12th Asian conference

on organic electronics, online, November 8−10, 2020.

Reference

1. S. Kim, H. J. Kwon, S. Lee, H. Shim, Y. Chun, W. Choi, J. Kwack, D. Han, M. Song, S. Mohammadi,

I. Kee and S. Y. Lee, Advanced Materials, 2011, 23, 3511-3516.

2. R. Asaki, S. Yokoyama, H. Kitagawa, S. Makimura, F. Abe, T. Yamazaki, T. Kato, M. Kanno, Y.

Onoyama, E. Hasegawa and K. Uchino, SID Symposium Digest of Technical Papers, 2014, 45, 219-222.

3. S.-H. K. Park, C.-S. Hwang, J.-I. Lee, S. M. Chung, Y. S. Yang, L.-M. Do and H. Y. Chu, SID

Symposium Digest of Technical Papers, 2006, 37, 25-28.

4. H.-L. Huang, T.-C. Chao, J. Sun and C.-J. Lin, SID Symposium Digest of Technical Papers, 2019, 50,

494-496.

5. C. Ganzorig and M. Fujihira, Applied Physics Letters, 2002, 81, 3137-3139.

6. H. Jung, S. Kang, H. Lee, Y. J. Yu, J. H. Jeong, J. Song, Y. Jeon and J. Park, ACS Applied Materials &

Interfaces, 2018, 10, 30022-30028.

Investigation of Oxide-Channel Ferroelectric-Gate Transistor

using Yttrium Doped Hafnium- Zirconium Oxide Fabricated by Solution Process

Doctoral Degree Tokumitsu Laboratory

Student number: 1620433 Student name: Mohit

1. Research Content 1.1. Background

Discovery of ferroelectricity in HfO2 has gain great interest to develop future high density ferroelectric random-access memory (FeRAM) devices. Ferroelectricity in HfO2 is due to the formation of orthorhombic phase. This phase is considered as metastable phase and can be stabilized by doping. The most widely used doping elements are Al, Si, Y and Zr. In particular, Zr doped HfO2 or Hafnium-Zirconium Oxide (HZO) has gained most significant attention because of high remnant polarization with wide doping range. Atomic layer deposition (ALD) and sputtering are usually used to fabricate HfO2. On the other hand, inexpensive devices can be fabricated by CSD method for various applications at low cost. However, there are only a few reports on CSD for ferroelectric HZO films and doping effect in HZO films by CSD has not been investigated. In this work, ferroelectric yttrium-doped hafnium-zirconium oxide (Y-HZO) has been realized by the chemical solution deposition (CSD) and applied to gate insulator of ferroelectric gate thin film transistor (FGT). Oxide semiconductor or conductor, In2O3 or In-Sn-O (ITO) was used as a channel, which was fabricated by CSD, too

1.2. Results and discussion At first, ferroelectric properties of Y-HZO films fabricated by CSD with various annealing temperature and environment was studied in chapter 3. Samples annealed at 600–800 °C in a vacuum, N2 and O2 environment showed ferroelectricity, which was confirmed by the polarization–electric field (shown in figure 1) and capacitance–voltage measurements. It was found that the ferroelectric properties were dramatically improved when the samples were annealed in a vacuum at 800 °C and the leakage current was decreased, compared to the Y-HZO films annealed at 800 °C in oxygen and nitrogen. Next, stability of ferroelectricity in HZO films deposited by sputtering and CSD has been investigated for oxide channel ferroelectric-gate thin film transistor application in chapter 4. Since it is known that the ferroelectric orthorhombic phase is metastable in HfO2-baseed ferroelectric materials and we need to deposit oxide

Figure 1: Electrical properties of Y-HZO annealed in various annealing environment

channel layer on the ferroelectric gate insulator followed by the annealing process, the robustness of the ferroelectricity is important to fabricate FGTs. After confirming the ferroelectricity of both sputtered HZO and CSD yttrium-doped HZO (Y-HZO) films, indium-tin-oxide (ITO) was deposited by sputtering on sputtered HZO or CSD Y-HZO layer to fabricate metal-ferroelectric-semiconductor (MFS) structure. Then, the MFS structures were re-annealed in N2 environment for 15 min to examine the robustness of ferroelectricity and electrical properties were measured as shown in figure 2 (a) and (b). Interestingly, it was found that the sputtered HZO films became paraelectric which was confirmed by both X-ray diffraction (XRD) pattern and electrical measurements. On the other hand, the CSD Y-HZO films showed ferroelectric nature even after the re-annealing with a negligible monoclinic phase.

Figure 2: Electrical properties of HZO after re-annealed at 600 oC (a) CSD Y-HZO (b) sputtered HZO. Next, ferroelectric gate transistor (FGT) with Y-HZO gate insulator and oxide channel with various thickness of In2O3 and ITO were fabricated and characterized in chapter 5. Figure 3(a) and (b) shows schematic illustration and photograph of FGT, where both Y-HZO and channel layers were fabricated by CSD. First, ferroelectric properties of vacuum and oxygen Y-HZO in the metal–ferroelectric–semiconductor (MFS) structure with 5-22 nm thick In2O3 and 6-24 nm thick ITO, have been confirmed by polarization-voltage (P-V) and capacitance-voltage (C-V) characteristics. The C–V curves showed clear butterfly loops showing the depletion of In2O3 and ITO layer. Vacuum annealed samples show good ferroelectric properties as compared to oxygen annealed samples. Secondly, the device performance of FGTs for both vacuum and oxygen annealed samples has been evaluated with various thickness of In2O3 and ITO channel layer as shown in figures 3(c) and (d). The fabricated FGTs exhibited typical n-channel transistor operation with a counterclockwise hysteresis loop in their transfer curves due to the ferroelectric nature of the Y-HZO-gate insulator. It was found that FGT shows low sub-threshold voltage swing (SS), high on/off drain current ratio of 106, large on current, and memory window for vacuum annealed sample. In contrast, FGTs with Y-HZO gate insulator annealed in oxygen show degraded device performance due to high leakage current. In this thesis, good electrical properties have been demonstrated for oxide channel FGTs with ferroelectric Y-HZO gate insulator fabricated by CSD with vacuum annealing process, which can be a suitable candidate for future low-cost ferroelectric device and memory applications.

2. Research Purpose The objective of this study is to develop the chemical solution process for HfO2-based ferroelectric films and to apply them to ferroelectric-gate transistors (FGTs). To realize FGTs, oxide semiconductor or conductor, In2O3 or In-Sn-O (ITO) was used as a channel and yttrium doped HZO was used as gate insulator material. To best of my knowledge, this is the first report of thin film transistors (TFTs) using solution processed HZO ferroelectric gate insulator with solution processed In2O3 or ITO oxide channel.

Figure 3: (a) schematic illustration (cross section), (b) photograph of FGT (top view), Id-Vg characteristics of FGT with (c) 5-nm-thick In2O3 channel and (d) 13-nm-thick ITO channel. 3. Research Achievements

A) Publications in peer-reviewed Scientific journals 1. Mohit, K. Haga, E. Tokumitsu, “Electrical properties of yttrium-doped hafnium-

zirconium dioxide thin films prepared by solution process for ferroelectric gate insulator TFT application”, Jpn. J. Appl. Phys., 59, pp. SMMB02-1-9, 2020.

2. Mohit, T. Murakami, K. Haga and E. Tokumitsu, “Impact of annealing environment on electrical properties of yttrium-doped hafnium-zirconium dioxide thin films prepared by the solution process” Jpn. J. Appl. Phys. 59 SPPB03, 2020.

3. Mohit, Takaaki Miyasako and Eisuke Tokumitsu, “Indium oxide and indium-tin-oxide channel ferroelectric gate thin film transistors with yttrium doped hafnium-zirconium dioxide gate insulator prepared by chemical solution process”, Jpn. J. Appl. Phys 60 SBBM02, 2021.

B) Manuscripts submitted or under preparation

1. Mohit, Shinji Migita, Hiroyuki Ota, Yukinori Morita and Eisuke Tokumitsu, “Stability of ferroelectricity in hafnium-zirconium dioxide films deposited by sputtering and chemical solution deposition for oxide-channel ferroelectric-gate transistor applications” Applied Physics Express (under review)

2. Mohit, Diasuke Hirose, Yuzuru Takamura and Eisuke Tokumitsu,“Development of novel DNA biosensor using HfO2 gate insulator and In2O3 channel BioTFTs prepared by solution process” (under preparation)

C) Conferences

International Conferences:

1. Mohit, Ken-Ichi Haga, Eisuke Tokumitsu, “Preparation of Ferroelectric Yttrium-doped Hafnium-Zirconium Dioxide Thin Films by Solution Process”, International Workshop on Dielectric Thin Films for Future Electron Devices: Science and Technology (IWDTF 2019), Tokyo, Japan, Nov 2019 (Poster)

2. Mohit, Takaaki Miyasako and Eisuke Tokumitsu, “Fabrication of Indium-Tin-Oxide Channel Ferroelectric-Gate Thin Film Transistors using Yttrium Doped Hafnium-Zirconium Dioxide by Chemical Solution Process”, SSDM, Toyama September 2020 (Oral).

3. Mohit, Shinji Migita, Hiroyuki Ota, Yukinori Morita and Eisuke Tokumitsu, “Robustness of ferroelectricity in hafnium-zirconium dioxide films deposited by sputtering and chemical solution deposition for ferroelectric transistor applications”, ECS PRiME, Hawaii, USA, October 2020. (Poster)

4. Mohit, Daisuke Hirose, Eisuke Tokumitsu and Yuzuru Takamura, “DNA biosensing using Indium Oxide thin film transistor with HfO₂ as gate insulator prepared by solution process”, E-MRS spring 2021, France, May 2021. (Oral-submitted)

Domestic Conferences:

1. Mohit, Yoshiaki Sumitomo, Ken-chi Haga, Keisuke Ohdaira, Eisuke Tokumitsu, “Sub-20 nm thick ferroelectric Hf-Zr-O films fabricated by solution process for ferroelectric-gate TFT applications”, Thin Film Materials & Devices Meeting (TFMD 2019), Kyoto, Japan, November 2019 (Poster).

2. Mohit, Jyotish Patidar, Ken-Ichi Haga, Eisuke Tokumitsu, “Fabrication of ferroelectric hafnium-zirconium dioxide thin films by solution process”, JSAP spring meeting 2020, Tokyo, March 2020 (Oral)

3. Mohit, Ken-Ichi Haga, Eisuke Tokumitsu, “Effect of Annealing Environment on Ferroelectric Properties of Hf-Zr-O (HZO) Thin Films Prepared by Solution Process”, Ferroelectric Meeting and Their Applications (FMA 37), Kyoto, Japan, May 2020 (Oral).

4. Mohit, Eisuke Tokumitsu, “Ferroelectric Properties of Hafnium-Zirconium-Dioxide Prepared by Chemical Solution Process for MFM and MFS Structures”, JSAP autumn meeting 2020, Kyoto, September 2020 (Oral)

5. Mohit, Shinji Migita, Hiroyuki Ota, Yukinori Morita and Eisuke Tokumitsu, Fabrication of ferroelectric gate thin film transistors using CSD Y-HZO and sputtered HZO with sputtered ITO channel JSAP spring meeting 2021, Tokyo, March 2020 (Oral-submitted)

D) Award and Research grant

Principal investigator for JAIST Research Grant (Fundamental Research) 2020 (on going).

Keywords: Ferroelectric Hafnium dioxide, oxide channel, Ferroelectric Gate Transistor, solution process, thin films.

1

Decoupling the Effect of Rubbing and Annealing on The Enhanced Device Performance in

Sequentially Deposited Bilayer Organic Solar Cells

(Materials Science)

Murata laboratory

ID: 1540208

Name: Mohd Zaidan bin Abdul Aziz

Organic solar cells (OSCs) have drawn tremendous attention as they possess several advantages such as

being semi-transparent, flexible, low cost, lightweight and processable on a large-scale at low

temperatures.1–6 Poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester

(PCBM) are considered as reference conjugated polymer electron donor and fullerene derivative electron

acceptor employed in OSC active layers.7,8 Although most active layers are fabricated using co-deposition

of the donor and acceptor materials into bulk heterojunctions (BHJs) thin films, sequential deposition of

diffusive P3HT/PCBM bilayers (BLs) provides the possibility to independently tune the properties of each

layer.9,10 BLs also offer the opportunity to form adequate donor/acceptor vertical concentration gradients

in active layers for regular architecture OSCs, which can facilitate charge percolation to the electrodes and

reduce leak currents.11 Rubbing P3HT prior to PCBM deposition has been demonstrated to enhance the

performances of annealed (AND) BL OSCs through the formation of such adequate donor-acceptor vertical

concentration profiles.12 Additionally, the application of pressure on the P3HT layer by uniaxial rubbing

has been reported to be an effective strategy to change the molecular orientation of P3HT molecules from

an edge-on to a face-on configuration.12,13

The performance of the optimized OSCs is aimed to be improved by adjusting the donor layer via rubbing

technique. Another objective of this research is to investigate the mechanism of the enhanced photovoltaic

performance of the rubbed OSCs through the morphological, compositional and structural studies.

Ultimately this research has been carried out to confirm whether the enhanced device performance should

be attributed to rubbing, annealing or a combination of both processes. The investigation of the individual

impacts of rubbing and thermal annealing on the vertical organization and molecular orientation of the

active materials in regular architecture BL OSCs has been performed extensively to gain in-depth

understanding on the major contributing factors on the enhanced OSCs performance.

Enhanced Photovoltaic Performance of P3HT/PCBM OSCs by Rubbing

Using advanced analytical techniques,

the origins of the enhanced PV

performance of as-deposited (ADP) and

AND bilayer P3HT/PCBM organic solar

cells (Figure 1a) upon P3HT layer

(electron donor) rubbing prior to PCBM

(electron acceptor) deposition have been

studied. Rubbing P3HT pior to PCBM

deposition (Fig. 1b) has been found to

enhance the PV performance of

P3HT/PCBM OSCs in both ADP and

AND conditions as shown the J-V curves

(Fig 1c). 5 times rubbing improved the

power conversion efficiency (PCE) of

the ADP OSCs from 0.83% up to 1.58%

in ADP OSCs. As for the AND OSCs, 10

times rubbing enhanced the PCE from 2.78% up to 3.09%.

Figure 1. Schematic representation of (a) BL OSCs (b) rubbing. (c)

J-V curves of ADP 0, 5 times rubbed and AND 0, 10 times rubbed

P3HT/PCBM OSCs.

(a)

(b)

(c)

2

Individual Effect of Rubbing and Annealing on P3HT/PCBM OSCs Performance

Rubbing has been found to locally remove P3HT from the surface of the deposited films where the

variations of the surface roughness and thickness of P3HT films may influence the degree of PCBM

diffusion into the underlying ADP P3HT layer. Energy-dispersive X-ray spectroscopy (EDS) and p-

polarized multiple-angle incidence resolution spectrometry (pMAIRS) are employed to measure the

concentration gradient and molecular orientation in the thin films, respectively. The compositional studies

(Fig. 2) of the ADP and AND devices performed via EDS suggest that adequate vertical profiles are

produced when P3HT films are rubbed 5 or 10 times. Fig 2b indicates that a layer with equally mixed P3HT

and PCBM (intermixed layer) is formed between 40 nm and 70 nm which is covered with a PCBM-rich

layer located between 20 nm and 40 nm. Such concentration profile enables efficient charge generation and

transport to the respective electrodes. Consequently, the Jsc and FF of the 5 times-rubbed ADPs increase

from 5.19 to 5.94 mA/cm2 and 0.43 to 0.48, respectively, compared to the reference unrubbed BL OSCs

(Fig. 2a). As shown in Fig. 2c, annealing leads to large improvements in PV performances of the BL OSCs

through the formation of a P3HT-free top interface which promotes efficient electron collection at the Al

electrode and induces crystallization of the materials. The active layers of Fig. 2d are composed of

intimately mixed electron donor and electron acceptor materials that further enhance the charge generation

efficiency within the active layer.14,15 These observations correlate well with the PV performance of AND

devices in which the absence of P3HT near the Al electrodes combined with the large intermixed layer

produce the highest Voc (0.63 V), Jsc (7.45 mA/cm2) and FF (0.66) in the 10 times-rubbed OSCs.

By controlling the diffusion of PCBM molecules into the underlying P3HT layer in diffusive BL OSCs, we

are able to fabricate ADP devices which exhibit a significant increase in PCE from 0.83% (unrubbed) to

1.58% (5 times-rubbed). In unrubbed devices, annealing results in the formation of a phase segregated

PCBM layer at the top interface of the photoactive layer, which brings about a substantial increase of PCE

from 0.83% (ADP) to 2.78% (AND). The combination of rubbing and thermal annealing further increases

the PCE of P3HT/PCBM BLs in regular architecture OSCs to 3.09% (10 times-rubbed). Our results clearly

demonstrate that rubbing the P3HT layer prior to PCBM deposition leads to sufficient vertical concentration

gradients in ADP BL P3HT/PCBM OSCs while annealing further enhances the concentration profiles of

the rubbed BLs by forming the equally mixed interlayers capped with PCBM buffer layers. Compositional

studies also explain the lower impact that rubbing has on the PV performances of AND OSCs as compared

to those of ADP devices.

Figure 3. The IR pMAIRS absorbance

spectra of ADP and AND P3HT/PCBM

films with different rubbing times on

P3HT films. The typical error bar for the

orientation angles is ±5°.

Figure 2. The cross-sectional EDS element mappings

of ADP (a) 0, (b) 5 times rubbed; and AND (c) 0, (d)

10 times rubbed P3HT/PCBM OSCs. The detected

elements are indium (blue), sulfur (green), carbon

(black) and aluminum (red). The white line represents

the normalized sulfur profile. The color scales above

the EDS images indicate the estimated active layer

composition based on the normalized sulfur profiles.

(a) 0x (ADP) (b) 5x

(c) 0x (AND) (d) 10x

3

To investigate the influence of rubbing on the molecular orientation at the surface of P3HT layer, pMAIRS

measurements are performed on P3HT films with increasing rubbing times. As indicated in Fig. 3, pMAIRS

spectra of the unrubbed ADP P3HT/PCBM films show the presence of strong ν(C=C) and (C-H) bands in

the IP spectrum, indicating an edge-on molecular orientation with a calculated orientation angle () of the

(C-H) mode from the surface normal of C-H =58°. Rubbing slightly aligns the P3HT molecules to C-H =

62.5° which is not a notable increment of C-H since pMAIRS typically generates measurement deviations

of ± 5°.16 Having the comparable C-H to those of corresponding P3HT single layers, the pMAIRS results

of the P3HT/PCBM BLs indicate that deposition of the PCBM layer does not significantly change the

molecular orientation in unrubbed P3HT thin films. After undergoing the annealing treatment, the band

intensities of the OP peaks remain almost unchanged regardless of the rubbing number. The small variations

of the indicate that the large FF enhancements in rubbed ADP and AND BL devices cannot be ascribed

to molecular reorientation in the P3HT layer.

Structural studies infer that the slight change of molecular orientation upon rubbing plays a less significant

role than the formation of adequate concentration gradients to be correlated with the PV performance of

BL devices. Despite the lack of edge-on to face-on molecular reorientation in P3HT/PCBM BLs, higher

FF can be measured in the rubbed AND devices compared to the rubbed ADP OSCs, which confirms that

adequate concentration profiles strongly impact the charge extraction properties of the devices. The

interfacial and bulk properties obtained from the morphological and compositional studies consequently

point out the importance of controlling the concentration gradient of the active layer to achieve high

performances in both ADP and AND BL OSCs. The characterizations carried out throughout the research

point out the significance of controlling the vertical concentration gradient of the photoactive layer in order

to realize the BL OSCs having the decent PV performances.

Research Originality:

The use of pMAIRS measurement and EDS spectroscopy to confirm whether the enhanced PV performance

of P3HT/PCBM OSCs is attributed to rubbing, annealing or combination of both processes.

Research Novelty:

Structural studies vis pMAIRS that complement the widely used GIXRD to evaluate the molecular

orientation of P3HT single layer and P3HT/PCBM BLs after rubbing.

Individual Effect of Rubbing and Annealing on P3HT/PCBM OSCs by characterizing the samples in

both as-deposited and annealed conditions.

Research Achievements

- Publication

1. Mohd Zaidan Abdul Aziz, Koichi Higashimine, Nobutaka Shioya, Takafumi Shimoaka, Takeshi

Hasegawa, Heisuke Sakai, Varun Vohra and Hideyuki Murata, “Controlling the concentration gradient

in sequentially deposited bilayer organic solar cells via rubbing and annealing”, RSC Advances, 2020,

10, 37529-37537.

- International Conferences

1. Mohd Zaidan, Heisuke Sakai, and Hideyuki Murata, “Effect of Varying Rubbing Pressures on Optical

Properties of Pristine Bilayer P3HT/PCBM Organic Photovoltaic Cells”, 9th International Conference

on Molecular Electronics and Bioelectronics (M&BE9), Kanazawa, Japan, 2017.06.26-28. (Poster

presentation).

4

2. Mohd Zaidan, Heisuke Sakai, and Hideyuki Murata, “Effect of rubbing on the performance of bilayer

P3HT/PCBM organic photovoltaic cells utilizing high regioregular P3HT”, 9th International

Conference on Nanoscience and Nanotechnology 2018 (NANO-SciTech 2018), Shah Alam, Selangor,

Malaysia, 2018.02.26-2018.03.01. (Oral presentation).

- Domestic Conference

1. Mohd Zaidan, Heisuke Sakai, and Hideyuki Murata, “Controlling Open-circuit Voltage and Fill Factor

through P3HT Regioregularity and Rubbing Technique”, The Japan Society of Applied Physics (JSAP)

Hokuriku Shin-Etsu Division Conference, Kanazawa, Japan, 2017.12.09. (Oral presentation).

- Award of Best Oral Presentation

1. Mohd Zaidan, Heisuke Sakai, and Hideyuki Murata, “Effect of rubbing on the performance of bilayer

P3HT/PCBM organic photovoltaic cells utilizing high regioregular P3HT”, 9th International

Conference on Nanoscience and Nanotechnology 2018 (NANO-SciTech 2018), Shah Alam, Selangor,

Malaysia, 2018.02.26-2018.03.01.

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