Recent Research

New Mo₆Te₆ Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe₂

Published on-line: 10 MARCH 2017

Hui Zhu, Qingxiao Wang, Chenxi Zhang, Rafik Addou, Kyeongjae Cho, Robert M. Wallace, and Moon J. Kim

A novel phase transition, from multilayered 2H-MoTe2 to a parallel bundle of sub-nanometer-diameter metallic Mo6Te6 nanowires (NWs) driven by catalyzer- free thermal-activation (400–500 °C) under vacuum, is demonstrated. The NWs form along the 〈11–20〉 2H-MoTe2 crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe₂ due to favorable band-alignment. In particular, an atomically sharp MoTe₂/Mo₆Te₆ interface and van der Waals gap with the 2H layers are preserved. The work highlights an alternative pathway for forming a new transition metal dichalcogenide phase and will enable future exploration of its intrinsic transportation properties.

http://dx.doi.org/10.1002/adma.201606264

Two-dimensional gallium nitride realized via graphene encapsulation

Zakaria Y. Al Balushi,^1,2 Ke Wang,³ Ram Krishna Ghosh,^4,5 Rafael A. Vilá,^1,2 Sarah M. Eichfeld,^1,2,3Joshua D. Caldwell,⁶ Xiaoye Qin,⁷ Yu-Chuan Lin,^1,2 Paul A. DeSario,⁶ Greg Stone,^1,3 Shruti Subramanian,^1,2 Dennis F. Paul,⁸ Robert M.Wallace,⁷ Suman Datta,^4,5 Joan M. Redwing^1,2,3,5 and Joshua A. Robinson^1,2,3

1 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.2 Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.3 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.4 Department of Electrical Engineering, University of Norte Dame, Notre Dame, Indiana 46556, USA.5 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.6 US Naval Research Laboratory,Washington DC 20375, USA.7 Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson,Texas 75080, USA.8 Physical Electronics USA, 18725 Lake Drive East, Chanhassen, Minnesota 55317, USA.

The spectrum of two-dimensional (2D) and layered materials ‘beyond graphene’ off.ers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (.5.0–6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices. A gap, however, remains between the theoretical prediction of 2D nitrides ‘beyond hBN’ and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.

http://dx.doi.org/10.1038/nmat4742

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.

DOI: 10.1021/acs.nanolett.6b01853

Covalent Nitrogen Doping and Compressive Strain in MoS₂ by

Remote N₂ Plasma Exposure

Angelica Azcatl,† Xiaoye Qin,† Abhijith Prakash,‡ Chenxi Zhang,† Lanxia Cheng,† Qingxiao Wang,†

Ning Lu,† Moon J. Kim,† Jiyoung Kim,† Kyeongjae Cho,† Rafik Addou,† Christopher L. Hinkle,†

Joerg Appenzeller,‡ and Robert M. Wallace*,†

†Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States‡Department of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette 47907, Indiana United States

Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low.The prototypical 2D material molybdenum disulfide (MoS₂) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS₂ monolayers by more than two orders of magnitude.The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8+/-0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

DOI: 10.1126/science.aad2114

27 NOVEMBER 2015 • VOL 350 ISSUE 6264 1065

Near-unity photoluminescence quantum yield in MoS₂

Matin Amani,^1,2* Der-Hsien Lien,^1,2,3,4* Daisuke Kiriya,^1,2* Jun Xiao,^5,2 Angelica Azcatl,⁶ Jiyoung Noh,⁶ Surabhi R. Madhvapathy,^1,2 Rafik Addou,⁶Santosh KC,⁶ Madan Dubey,⁷ Kyeongjae Cho,⁶ Robert M. Wallace,⁶ Si-Chen Lee,⁴

Jr-Hau He,³ Joel W. Ager III,² Xiang Zhang,^5,2,8Eli Yablonovitch,^1,2 Ali Javey^1,2†

¹Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. ²Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. ³Computer, Electrical and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST),Thuwal 23955-6900, Saudi Arabia. ⁴Department of Electrical Engineering, Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China. ⁵National Science Foundation Nanoscale Science and Engineering Center, University of California, Berkeley, Berkeley, CA 94720, USA. ⁶Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA. ⁷Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, MD 20723, USA. ⁸Department of Physics, King Abdulaziz University, Jeddah 21589, Saudi Arabia. *These authors contributed equally to this work.

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the

direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide

(MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant

tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

DOI: 10.1038/ncomms8311

Vol. 6, 7311 (2015)

Received 11 Dec 2014 | Accepted 27 Apr 2015 | Published 19 Jun 2015

Atomically thin resonant tunnel diodes built from

synthetic van der Waals heterostructures

Yu-Chuan Lin¹, Ram Krishna Ghosh², Rafik Addou³, Ning Lu³, Sarah M. Eichfeld¹, Hui Zhu³, Ming-Yang Li⁴,

Xin Peng³, Moon J. Kim³, Lain-Jong Li⁵, Robert M. Wallace³, Suman Datta² & Joshua A. Robinson¹

¹ Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park,Pennsylvania 16802, USA. ² Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ³ Departmentof Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA. ⁴ Institute of Atomic and Molecular Sciences,Academia Sinica, Taipei 10617, Taiwan. ⁵ Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.

APPLIED PHYSICS LETTERS 105, 141604 (2014)

A crystalline oxide passivation for Al₂O₃/AlGaN/GaN

Xiaoye Qin, Hong Dong, Jiyoung Kim, and Robert M. Wallace

Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA

(Received 8 July 2014; accepted 29 September 2014; published online 9 October 2014)

In situ X-ray photoelectron spectroscopy and low energy electron diffraction are performed to study the formation of a crystalline oxide on the AlGaN surface. The oxidation of the AlGaN surface is prepared by annealing and remote N₂:O₂ plasma pretreatments resulting in a stable crystalline oxide. The impact of the oxide on the interface state density is studied by capacitance voltage (C-V) measurements. It is found that a remote plasma exposure at 550 °C shows the smallest frequency dispersion. Crystalline oxide formation may provide a novel passivation method for high quality AlGaN/GaN devices.

[http://dx.doi.org/10.1063/1.489764]

Vol. 8, No.6 6265-6272

Hole Contacts on Transition Metal Dichalcogenides: Interface Chemistry and Band Alignments

Stephen McDonnell , Angelica Azcatl , Rafik Addou ,Cheng Gong , Corsin Battaglia†‡§, Steven Chuang†‡§,Kyeongjae Cho , Ali Javey†‡§, and Robert M. Wallace

†Electrical Engineering and Computer Sciences and§Berkeley Sensor and Actuator Center, University of California, Berkeley, California 94720, United States‡ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,United StatesDepartment of Materials Science and Engineering,The University of Texas at Dallas, Richardson, Texas 75080, United States

MoOx shows promising potential as an efficient hole injection layer for p-FETs based on transition metal dichalcogenides. A combination of experiment and theory is used to study the surface and interfacial chemistry as well as the band alignments for MoOx/MoS2 and MoOx/WSe2 heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy and density functional theory. A Mo5+ rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoOx contacts on MoS2 and WSe2.

[http://dx.doi.org/10.1021/nn501728w]

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

Nano Lett., 2014, 14 (3), pp 1337–1342

MoS2 P-type Transistors and Diodes Enabled by High Work Function MoOxContacts

Steven Chuang †‡§, Corsin Battaglia †‡§, Angelica Azcatl , Stephen McDonnell , Jeong Seuk Kang †‡§, Xingtian Yin †‡§, Mahmut Tosun †‡§, Rehan Kapadia †‡§, Hui Fang †‡§, Robert M. Wallace , and Ali Javey *†‡§

[http://dx.doi.org/10.1021/nl4043505]

Using an ultrathin (15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of 6.6 eV, which is significantly higher than

Nano Lett., 2014, 14 (2), pp 967–971

Hole Selective MoOx Contact for Silicon Solar Cells

Corsin Battaglia †‡, Xingtian Yin †‡§, Maxwell Zheng †‡, Ian D. Sharp , Teresa Chen , Stephen McDonnell , Angelica Azcatl , Carlo Carraro #, Biwu Ma , Roya Maboudian #, Robert. M. Wallace , and Ali Javey *†‡

† Electrical Engineering and Computer Sciences Department, University of California, Berkeley, California 94720, United States‡ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,United States§ Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an, 710049 Shaanxi, People’s Republic of ChinaJoint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United StatesMolecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United StatesMaterials Science and Engineering, University of Texas, Dallas, Texas 75083, United States# Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States

that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

[http://dx.doi.org/0.1021/nl404389u]

The effect of room temperature ultraviolet-ozone (UV-O3) exposure of MoS2 on the uniformity of subsequent atomic layer deposition of Al2O3 is investigated. It is found that a UV-O3 pre-treatment removes adsorbed carbon contamination from the MoS2 surface and also functionalizes the MoS2 surface through the formation of a weak sulfur-oxygen bond without any evidence of molybdenum-sulfur bond disruption. This is supported by first principles density functional theory calculations which show that oxygen bonded to a surface sulfur atom while the sulfur is simultaneously back-bonded to three molybdenum atoms is a thermodynamically favorable configuration. The adsorbed oxygen increases the reactivity of MoS2 surface and provides nucleation sites for atomic layer deposition of Al2O3. The enhanced nucleation is found to be dependent on the thin film deposition temperature.

APPLIED PHYSICS LETTERS 104, 111601 (2014)

MoS₂ functionalization for ultra-thin atomic layer deposited dielectrics

Angelica Azcatl, Stephen McDonnell, Santosh K. C., Xin Peng, Hong Dong, Xiaoye Qin, Rafik Addou, Greg I. Mordi, Ning Lu, Jiyoung Kim, Moon J. Kim, Kyeongjae Cho, and Robert M. Wallace

Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, USA(Received 28 December 2013; accepted 9 March 2014; published online 19 March 2014)

[http://dx.doi.org/10.1063/1.4869149]

Vol.8 No. 3 2880-2888 (2014)

Defect-Dominated Doping and Contact Resistance in MoS₂

Stephen McDonnell, Rafik Addou, Creighton Buie, Robert M. Wallace, and Christopher L. Hinkle

ABSTRACT Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS₂ dominate the metal/MoS₂contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS₂ can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS₂ since the same metal in contact with MoS₂ can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.

DOI: 10.1021/nn500044q