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CXXXX American Chemical Society

Interfacial Nondegenerate Dopingof MoS2 and Other Two-DimensionalSemiconductorsSanjay Behura and Vikas Berry*

Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States

Carrier confinement by isolating sin-gle sheets from layered materialshas led to the realization of a wide

variety of extraordinary phenomena andapplications. This outcome was first recog-nized in graphene, where, due to the na-scent massless Fermions, ballistic carriertransport was achieved. The field has sinceembraced two-dimensional semiconduc-tors (2DSs), with molybdenum disulfide(MoS2) being the most studied structure.Monolayer MoS2, with a stratum of molyb-denum atoms sandwiched between twolayers of sulfur atoms in a trigonal prismatic(or antiprismatic) lattice, is a direct band gaptwo-dimensional (2D) material (∼1.8 eV).With a flexible, ultrathin, semiconducting,and photoluminescent structure,1 MoS2'sversatile functionality is being integratedwith other 2D nanomaterials (2DNs) to ac-cess new physics. To expand the scope of itsapplications, it is imperative to control 2DSs'Fermi level, which can enable defined car-rier transitions and heterojunction-inducedband bending. The applications that will beimpacted include optoelectronic devices,2

catalysis,3 memory devices,4 Schottky cir-cuits,5 controlled photoluminescence,1 andmolecular sensing.Similar to three-dimensional (3D) semi-

conductors, the doping density and theposition of the Fermi level in 2DNs will

influence their electrical and optical prop-erties.6 While doping in 3D semiconductorssuch as silicon is achieved via ion implanta-tion, such methods cannot be applied to-ward 2DNs because it leads to significantdefect density and deterioration of 2DNproperties.7 This is because 2DNs have largesurface areas, which when exposed to ionsproduce more defects per unit volume.8

Further, several doping mechanisms onMoS2 have produced degenerate doping,which leads to MoS2 behaving like a metalwith the resultant Fermi level close to theedge of the conduction band (or valenceband).9 Although degenerate doping is im-portant to tune the Schottky barrier withmetal contacts,9 nondegenerate dopingcan provide discrete energy levels withinthe forbidden band gap, thus providing anavenue for optoelectronic transitions and/orcarrier recombination processes (Figure 1). Inthe current issue of ACS Nano, Park et al.10

demonstrate a unique and effective mech-anism to proxy-dope 2DSs controllably inthe nondegenerate regime via an inter-faced, dopant-rich phosphorus silicate glass(PSG) substrate. The process involves en-hancing the intimacy between MoS2 andthe dopants (P2O5) via (1) dopant out diffu-sion between 700 and 900 �C, (2) thermalactivation at 500 �C, and (3) optical activationabove 5 μW laser irradiation (Figure 2).10

* Address correspondence tovikasb@uic.edu.

Published online10.1021/acsnano.5b01442

ABSTRACT Controlled nondegenerate doping of two-dimensional semiconductors (2DSs) with their

ultraconfined carriers, high quantum capacitance, and surface-sensitive electronics can enable tuning their

Fermi levels for rational device design. However, doping techniques for three-dimensional semiconductors,

such as ion implantation, cannot be directly applied to 2DSs because they inflict high defect density. In this

issue of ACS Nano, Park et al. demonstrate that interfacing 2DSs with substrates having dopants can

controllably inject carriers to achieve nondegenerate doping, thus significantly broadening 2DSs'

functionality and applications. Futuristically, this can enable complex spatial patterning/contouring of

energy levels in 2DSs to form p�n junctions, integrated logic, and opto/electronic devices. The process is

also extendable to biocellular-interfaced devices, band-continuum structures, and intricate 2D circuitry.

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Here, the asymmetry-induced di-pole moment of P2O5 with negativecharge on the O atoms attractsholes to n-dope MoS2 at the inter-face. During the optical exposure,the laser's photon energy overthe band gap (hν � Eg) producesequilibrium heat at the PSG�MoS2interface to activate n-doping. Theincrease in the dopant's weight per-centage in the substrate increasesMoS2 doping, while the thickness ofMoS2 does not influence the dopingprocess.The molecular doping mecha-

nism for 2DSs can be influenced by(a) the dipole moment of the inter-facing molecule (molecular inter-faces influence doping on 2DNs11),(b) the quantum capacitance of the2DSs, (c) the dipolar orientation anddistance, and (d) molecular bond-ing and/or orbital rehybridization.

The molecular dipole moment willbe associated with an electric field,which will induce doping on 2DSsbased on its quantum capacitance:CQ = δQ/δφs, whereQ is the channelcharge and φs is the surface poten-tial from the interfacing molecule.Clearly, the orientation of moleculardipoles and their distance from the2DSs influence the electric field im-pinging on the 2DS and, therefore,the level of doping.

OUTLOOK AND FUTURECHALLENGES

The process to introduce nonde-generate doping in MoS2 via inter-faced substrates can be extended tomost of the 2DNs, including MoSe2,WS2, WSe2, and WTe2. However,since the process involves thermal an-nealing, the chemical stability of 2DNs(MoSe2 ≈ 980 �C, WS2 ≈ 1040 �C,

WSe2 ≈ 930 �C, WTe2 ≈ 700 �C) athigher temperature will govern thepossible extent of doping. It is im-portant to note that 2DSs (and2DNs) are essentially “surfaces”,and therefore, their interface ismore sensitive. This is because aninterface will influence a larger vol-ume of 2DSs in comparison to asimilar interface on 3D semiconduc-tors' surfaces. Moreover, researchwith other substrate molecules ofvaried dipole moments is essen-tial to determine if nondegeneratedoping can be achieved withoutthermal or optical annealing. Theprocess can also be employedto study the interfacial potentialchange in biological cells interfacedwith 2DSs. Here, the resultantchange in doping (captured via

Raman spectroscopy) can provide in-formation on cell surface reactions

Figure 1. Schematicof theeffectof interfacingadopedsubstratewitha two-dimensional semiconductor (2DS)on itsband structure.The dipole moment of the molecule induces doping, which is proportional to the quantum capacitance of the 2DS. The molecularorientationand thedistance fromthe2DS influencedopingdensity. Thefigurealso showspossible doping fromthe top faceof 2DSs.

Figure 2. (a) Three-dimensional semiconductors such as silicon can be doped via ion implantation. (b) Park et al. show thattwo-dimensional semiconductor (2DS) can be doped via an interfaced substratewith imbedded dopantmolecules.10 This canbe achieved by a two-step thermal process or a three-step thermal�optical process.

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and surface potential (due to actua-tion of ion channels).A spatially contrasting doping

pattern on the substrate can beemployed to introduce selective non-degenerate doping levels and/or de-generate doping regions (Figure 3).For example, a 2DS can be depos-ited on a substrate with patterneddopants, which can induce spatiallypolarized carrier densities and con-trasting Fermi levels to producecomplex electronic/optoelectronicdevices with predefined band struc-tures. The process proposed by Parket al.10 can be integrated with semi-conductor processing techniques(thermal doping, lithographic pat-terning, epitaxy, atomic layer de-position, plasma functionalization,chemical vapor deposition, andrapid thermal annealing) for analo-gous device fabrication, bringing2DS technology closer to practicalrealization.As an example, a design of aMoS2

p�n junction is presented inFigure 3 (top), where lateral polaritycontrast is achieved via selectivep- and n-type doping of the sub-strate interfaced with 2DSs. Thisconstruct can be applied for diodic

transistors, tunable optical responsedevices, or molecular sensors. Arough design of a substrate patternwith degenerate doped islandswithin the 2DSs' matrix doped inthe nondegenerate regime is shownin Figure 3. Here, Coulomb block-ade, electron tunneling, negativedifferential resistance, and/or ther-mionic emission for 2DS islandscan be investigated. Moreover,band continuum via gradient dop-ing can be employed for devicesresponding to a broad optical range(such as solar cells) or for electro-kinetic fluid flow.Finally, it is critical to account for

or to study the quantum capaci-tance of the 2DSs and their hetero-structures because CQ representsthe interaction between the molec-ular dipole moment and the resul-tant doping induced in 2DSs.Strategies have to be developed toenhance the mobility by employingsubstrates (doped) without dan-gling bonds, as they introduce scat-tering sites. Further, it is well-knownthat the 3D semiconductors under-go a slight reduction in band gapupon doping.12 This phenomenonis still to be characterized for 2DSs,

where the interface-induced dop-ing process can be effectively lever-aged. Another mechanism that re-quires further research is the electro-magnetic interaction-induced pho-non scattering near impurities in2DSs, which is known to modifythe band structures of 3D semicon-ductors.13 In conclusion, interface-induced proxy-doping of 2DSs canbe employed to control dopingdensity and Fermi level. Further,patterning the substrate with polar-ized dopants or dopant gradientscan produce unique electronic ar-chitectures for a wide range of sys-tems, including optoelectronics,biocellular interfacing, potential in-tegrated circuits, and logic devices.

Conflict of Interest: The authorsdeclare no competing financial interest.

Acknowledgment. V.B. acknowledgesfinancial support from National ScienceFoundation (Grants CMMI-1503681 andCMMI-1030963) and University of Illinoisat Chicago.

REFERENCES AND NOTES1. Splendiani, A.; Sun, L.; Zhang, Y.; Li,

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Figure 3. Patterned or gradient doping of the substrate can be employed to produce complex two-dimensional deviceconstructs. (Top left) By patterning n-type and p-type dopants on a substrate, a p�n junction can be achieved on theinterfaced two-dimensional semiconductor (2DS) (top right). (Bottom left) With patterning, the substrate high conductivity(high σ) island array can be realized within doped 2DSs (see top view in bottom right). These devices can be employed for aCoulomb blockade, electron tunneling, and/or thermionic emission studies.

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