Experimentally Synthesized 2D Materials at Rice University

Molybdenum disulfide (MoS₂)

Y. Zhan et al. “Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate”, Small, 2012, 8, No. 7, 966–971

Two prominent Raman modes - symmetric in-plane E¹_2g and out-of-plane A_1g vibrations.

 (e) shows HRTEM image with Moiré pattern whose FFT reconstruction in (f) reveals triple layer stack. (g) & (h) show SAED indicating hexagonal symmetry and polycrystallinity.

Molybdenum diselenide (MoSe₂)

X. Wang et al, “Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe₂”, ACS Nano, Vol.8, No. 5, pp. 5125 – 5131, 2014.

 Single crystalline monolayer flakes exhibiting large spatial extent and hexagonal symmetry.

Raman active vibrations similar to MoS2 but different energies. Direct band gap ~ 1.53 eV.

(c) shows high-res STEM image of 2H MoSe2 phase. Intensity map shows alternating Mo & Se signals (d) shows STEM-HAADF of single and bi-layer flakes.

Tungsten disulfide (WS₂)

Y. Gong et al. “Tellurium-Assisted Low-Temperature Synthesis of MoS2 andWS2 Monolayers”, ACS Nano, Vol. 9, No. 12, pp. 11658 – 11666, 2015.

(a) shows experimental Raman spectrum of WS2 (similar to MoS2) with theoretical predictions and fitting (b) shows direct band-gap in PL spectrum at ~ 2 eV.

HRTEM image of WS2 monolayer with FFT inset showing hexagonal symmetry.

Molybdenum disulfide/ tungsten disulfide heterostructures (MoS₂/ WS₂)

Y. Gong et al. “Vertical and in-plane heterostructures from WS2/MoS2 monolayers”, Nature Materials, Vol. 13, pp. 1135 – 1142, 2014.

Schematics and optical images of the vertical and in-plane monolayer heterostructures differentiable via contrast.

(left) Raman spectra from bottom MoS2 layer (1,2) and central MoS2/WS2 heterostructure (3.4) (right) PL spectra of (1,2) showing MoS2 direct A exciton (~1.85 eV, 680 nm) and (3.4) showing MoS2 and WS2 (~1.97 eV, 630 nm) A excitons and low energy interlayer exciton (~1.42 eV, 875 nm).

STEM image of central bilayer heterostructure showing Mo, W, and S signals (Z-contrast).

Molybdenum diselenide/ tungsten diselenide heterostructures (MoSe₂/ WSe₂)

Y. Gong et al. “Two-Step Growth of Two-Dimensional WSe2/MoSe2 Heterostructures”, Nano Letters, Vol. 15, pp. 6135 – 6141, 2015.

Optical images of (left) pristine MoSe2 monolayer, and Type 1 (center) and Type 2 (right) MoSe2/WSe2 bilayer heterostructures.

Illustrations of pristine MoSe2 (left) and the two types of grown bilayer heterostructures.

Raman spectra (center) collected from different points (1-3) shown on left assigned to MoSe2 and WSe2 lattice vibrations. PL spectra (right) from the same regions showing individual direct band-gaps at 1.51 eV (MoSe2), 1.58 eV (WSe2) and low energy interlayer exciton at 1.33 eV.

STEM image (center) of the MoSe2/ WSe2 bilayer interface with different contrast. Atomic reconstruction (right) showing the distinct lattices of the heterostructure.

Molybdenum sulfide selenide (MoS_2(1-x)Se_2x)

Y. Gong et al. “Band Gap Engineering and Layer-by-Layer Mapping of Selenium- Doped Molybdenum Disulfide”, Nano Letters, Vol. 14, pp. 442 – 449, 2014.

Raman spectrum (left) of an alloy composition showing characteristic MoS2 and MoSe2 lattice vibrations. PL spectra (right) as a function of atomic concentration "x" showing tunability from 1.5 eV to 1.8 eV.

(left) STEM-ADF image of a monolayer alloy (right) structural model showing atomic sites populated with Mo (red), S2 (dark green), Se+S (bright green), and Se2 (white).

Rhenium disulfide (ReS₂)

K. Keyshar et al. “Chemical Vapor Deposition of Monolayer Rhenium Disulfide (ReS2)”, Advanced Materials, Vol. 27, pp. 4640 – 4648, 2015.

Optical images showing variety of 2D morphologies and thicknesses of ReS2 crystals.

(left) Raman spectra showing wealth of active vibrational modes due to lower symmetry of distorted 1T crystal structure (right) PL spectra showing retention of direct band-gap but redshift with decreasing thickness.

 Z-contrast STEM image showing 1T' crystal structure. Inset shows larger area.

Tin disulfide (SnS2)

Gonglan Ye et al. “Synthesis of large-scale atomic-layer SnS2 through chemical vapor deposition”, Nano Research, Vol. 5, Issue C, pp. 1 – 9, 2017.

STEM high-res image of bilayer SnS2 crystal.

Vanadium disulfide (VS₂)

J. Yuan et al. “Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction”, Advanced Materials, Vol. 27, pp. 5605 – 5609, 2015.

SEM images showing hexagonal VS2 layers.

(left) Raman spectra of VS2 as a function of laser power showing prominent A1 and E2 modes (right) PL spectra as a function of temperature showing no appreciable band-gap.

HRTEM image of VS2 sheet with SAED pattern (inset).

Indium selenide (InSe)

S. Lei et al. “Evolution of the Electronic Band Structure and Efficient Photo-Detection

in Atomic Layers of InSe”, ACS Nano, Vol. 8, No. 2, pp. 1263 – 1272, 2014.

CVT-grown InSe exfoliated on top of SiO2 substrate showing monolayer (purple) and few layer (blue) regions.

Raman spectra as a function of layer thickness showing evolution of the mode intensities and frequencies.

HRTEM image of monolayer InSe and corresponding SAED pattern with hexagonal symmetry.

Gallium selenide (GaSe)

S. Lei et al. “Synthesis and Photoresponse of Large GaSe Atomic Layers”, Nano Letters, Vol. 13, pp. 2777 – 2781, 2013.

Optical images of Vapor phase mass transport (VMT) - grown GaSe monolayers with various morphologies.

Raman spectra as a function of layer thickness showing significant intensity change of A¹_1g mode and shift in A²_1g mode (right).

TEM image:

HRTEM image of [100] plane of GaSe showing lattice constant value and SAED pattern.

Hexagonal boron nitride (h-BN)

Z. Liu et al. “Ultrathin high-temperature oxidation-resistant coatings of hexagonal boron nitride”, Nature Communications, Vol. 4, pp. 1 – 8, May 2013.

(a) Photograph of h-BN grown on Ni foil (b-d) Optical images of bulk, few-layer, and bilayer h-BN films respectively (2 mm scale bars).

Variation in E_2g Raman mode of h-BN as a function of layer thickness.

(e-h) TEM images of bi-, tri-, quad-, and multi-layered h-BN films (i) STEM-ADF image of bilayer h-BN showing hexagonal symmetry in SAED (inset).