Electronic Materials: Physics and Applications (2015)Junqiao Wu Research Group

Department of Materials Science and Engineering, University of California, Berkeley

Materials Sciences Division, Lawrence Berkeley National Laboratory

http://mse.berkeley.edu/~jwu

Introduction

The Wu Group explores new applications and fundamental

physics of low-dimensional materials, layered transition metal

dichalcogenides (TMDs), strongly correlated materials and

their interfaces. We aim to understand the influence of

defects, doping and external stimuli on the electronic and

structural properties of these materials, for the ultimate goal

of applications in thermoelectrics, photovoltaics, memory,

NEMS relays, actuators, infrared detectors, and chemical

sensors.

Micro-Actuation via Structural Phase Transition

Growth and Doping of TMDs

We demonstrate stable p-type conduction in MoS2 by

substitutional niobium (Nb) doping, leading to a degenerate

hole density of ∼3×1019 cm−3. Structural and X-ray

techniques reveal that the Nb atoms are indeed

substitutionally incorporated into MoS2 by replacing the Mo

cations in the host lattice. van der Waals p−n homojunctions

based on vertically stacked MoS2 layers are fabricated, which

enable gate-tunable current rectification.

Synthesis & HRTEM characterization

Gate-tunable current rectification

Alloying of TMDs

Engineering Phase Transition in Nanomaterials

The abrupt first-order metal–insulator phase transition in

single-crystal vanadium dioxide nanowires (NWs) is

engineered to be a gradual transition by axially grading the

doping level of tungsten. Each individual NW acts as a

microthermometer that can be simply read out with an optical

microscope. This novel phase transition yields an extremely

high temperature coefficient of resistivity ∼10%/K,

simultaneously with a very low resistivity down to 0.001 Ω·cm,

making these NWs promising infrared sensing materials for

uncooled microbolometers. Lastly, they form bimorph thermal

actuators that bend with an unusually high curvature, ∼900

m–1·K–1 over a wide temperature range (35–80 °C),

significantly broadening the response temperature range of

previous VO2 bimorph actuators.

High Pressure Physics of Materials

Diamond anvil cells (DAC) can be used to apply hydrostatic

pressures up to 50 GPa (~ 500k atm); under these pressures,

most solid materials would exhibit new properties or

structures that do not exist at ambient pressure. For example,

the metal-insulator transition of VO2 can be induced by

hydrostatic pressure. We observed the phase transition of

single crystal VO2 beams in a DAC by Raman spectroscopy

and optical microscopy. The P-T phase diagram of VO2 was

constructed, which paves the way to potential applications.

Tuning Interlayer Coupling in TMDs

Synergy from Interfacing Dissimilar Materials

Acknowledgments

Optical images and PL mapping data taken on monolayers of (a) MoSe2

(x=0), (b) Mo0.86W0.14Se2 (x=0.14), (c) Mo0.25W0.75Se2 (x=0.75), and (d)

WSe2 (x=1). Scale bar is 5 μm.

Monolayer Mo1-xWxSe2 (0 < x < 1) alloys were experimentally

realized from synthesized crystals. Mo1-xWxSe2 monolayers

are direct bandgap semiconductors displaying high

luminescence and are stable in ambient. The bandgap values

can be tuned by varying the W composition. Results introduce

monolayer Mo1-xWxSe2 alloys with different gap values, and

open a venue for broadening the materials library and

applications of two-dimensional semiconductors.

S. Tongay et al. Appl. Phys. Lett. 104, 012101 (2014) K. Liu et al. Adv. Mater. 26 1746(2014)

By utilizing the 1% lattice expansion in VO2’s structural phase

change at 68oC, we build bending and torsional actuators.

H. Guo et al. Nature Comm., 5, 4986 (2014)

S. Lee et al. J. Am. Chem. Soc. 135, 4850 (2013)

Z. Tao et al. Phys. Rev. Lett., 109, 166406 (2012)

Y. Chen et al. in preparation (2015)

S. Tongay et al. Nature Comm. 5, 3252 (2014)

Large-area CVD-grown WS2/MoS2 heterostructures

Interlayer interaction is externally tuned from non-coupling to

strong coupling. Following this trend, the luminescence

spectrum of the heterostructures evolves from an additive line

profile where each layer contributes independently to a new

profile that is dictated by charge transfer and band

normalization between the WS2 and MoS2 layers.

S. Tongay et al. Nano Lett. 14, 3185 (2014)

X. Hong et al. Nature Nanotech. 9, 682 (2014)

PZT

WSe2

P↑P↓

PZT

SRO

STO

2D

Non-volatile memory devices exploiting heterostructures

interfacing 2D TMD crystals with ferroelectric oxide thin films

are demonstrated. A-few-unit-cell-thick TMD layers are

operated as semiconductor channels in ferroelectric field

effect transistors with low switching voltage, high on-off

current ratio, and reasonably good data retention and

endurance.C. Ko et al. in preparation (2015)

This work was supported by the National

Science Foundation (NSF), Department of

Energy (DOE), Singapore - Berkeley

Research Initiative on Sustainable Energy

(SinBeRISE), and Samsung Inc., etc.

Phase-transition activated microscale actuators are ultra-

high-performance, micron-scale devices that convert energy

from light, heat, or electricity into mechanical motion.

Designed to bend, flex, flap, or oscillate when energized, they

can become the motors, switches, pumps, and valves of

microfluidic systems. Fashioned into coils, they can rotate as

fast as 200,000 rpm without breaking. Such microscopic

bending and torsional machines could act as the mechanical

building blocks of artificial muscle.

K. Wang et al. ACS Nano 7, 2266 (2013)

a) Phase transition of VO2 beams observed

from optical images (P=13.6 GPa). The scale

bar is 10 um. b) Raman spectrum of VO2 at

different temperatures. (P=12.9 GPa).

Diamond anvil cell

P-T phase diagram of single crystal

VO2 beams (3 different samples).

Pressure transmitting media was 4:1

methanol-ethanol.

Thermal Conductivity of Nanostructures

By controlling the heat flow through a nanowire and

measuring the temperature change, we can find its thermal

conductivity. For thin films, we measure thermal conductivityvia 3ω method and time domain thermoreflectance (TDTR).

S. Lee, H. Choe in preparation (2015)

Ultrathin Eutectic Behavior

In ultrathin layers, the AuSi alloying rate can be enhanced

over 20x as the Au thickness decreases from 300nm to 20nm.

T. Matthews et al. Phys. Rev. Lett. 108, 096102 (2012)

J. Suh et al. Nano Lett. 14, 6976 (2014)