Honors & Awards
2015 Chinese Government Award for Outstanding Students Abroad, China Scholarship Council (Mar 2016)
Trinity College Graduate Academic Prize, Trinity College, University of Oxford (Oct 2014)
Doctor of Philosophy, University of Oxford, Materials (2016)
Bachelor & Master of Engineering, Imperial College London, Materials Science and Engineering (2012)
Hongjie Dai, Postdoctoral Faculty Sponsor
Revealing Defect-State Photoluminescence in Monolayer WS2 by Cryogenic Laser Processing
2016; 10 (6): 5847-5855
Understanding the stability of monolayer transition metal dichalcogenides in atmospheric conditions has important consequences for their handling, life-span, and utilization in applications. We show that cryogenic photoluminescence spectroscopy (PL) is a highly sensitive technique to the detection of oxidation induced degradation of monolayer tungsten disulfide (WS2) caused by exposure to ambient conditions. Although long-term exposure to atmospheric conditions causes massive degradation from oxidation that is optically visible, short-term exposure produces no obvious changes to the PL or Raman spectra measured at either room temperature or even cryogenic environment. Laser processing was employed to remove the surface adsorbents, which enables the defect states to be detected via cryogenic PL spectroscopy. Thermal cycling to room temperature and back down to 77 K shows the process is reversible. We also monitor the degradation process of WS2 using this method, which shows that the defect related peak can be observed after one month aging in ambient conditions.
View details for DOI 10.1021/acsnano.6b00714
View details for Web of Science ID 000378973700027
View details for PubMedID 27295362
Biexciton Formation in Bilayer Tungsten Disulfide
2016; 10 (2): 2176-2183
Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors, and their 2D structure results in large binding energies for excitons, trions, and biexcitons. The ability to explore many-body effects in these monolayered structures has made them appealing for future optoelectronic and photonic applications. The band structure changes for bilayer TMDs with increased contributions from indirect transitions, and this has limited similar in-depth studies of biexcitons. Here, we study biexciton emission in bilayer WS2 grown by chemical vapor deposition as a function of temperature. A biexciton binding energy of 36 ±4 meV is measured in the as-grown bilayer WS2 containing 0.4% biaxial strain as determined by Raman spectroscopy. The biexciton emission was difficult to detect when the WS2 was transferred to another substrate to release the stain. Density functional theory calculations show that 0.4% of tensile strain lowers the direct band gap by about 55 meV without significant change to the indirect band gap, which can cause an increase in the quantum yield of direct exciton transitions and the emission from biexcitons formed by two direct gap excitons. We find that the biexciton emission decreases dramatically with increased temperature due to the thermal dissociation, with an activation energy of 26 ± 5 meV. These results show how strain can be used to tune the many-body effects in bilayered TMD materials and extend the photonic applications beyond pure monolayer systems.
View details for DOI 10.1021/acsnano.5b06678
View details for Web of Science ID 000370987400056
View details for PubMedID 26761127
Doping Graphene Transistors Using Vertical Stacked Monolayer WS2 Heterostructures Grown by Chemical Vapor Deposition
ACS APPLIED MATERIALS & INTERFACES
2016; 8 (3): 1644-1652
We study the interactions in graphene/WS2 two-dimensional (2D) layered vertical heterostructures with variations in the areal coverage of graphene by the WS2. All 2D materials were grown by chemical vapor deposition and transferred layer by layer. Photoluminescence (PL) spectroscopy of WS2 on graphene showed PL quenching along with an increase in the ratio of exciton/trion emission, relative to WS2 on SiO2 surface, indicating a reduction in the n-type doping levels of WS2 as well as reduced radiative recombination quantum yield. Electrical measurements of a total of 220 graphene field effect transistors with different WS2 coverage showed double-Dirac points in the field effect measurements, where one is shifted closer toward the 0 V gate neutrality position due to the WS2 coverage. Photoirradiation of the WS2 on graphene region caused further Dirac point shifts, indicative of a reduction in the p-type doping levels of graphene, revealing that the photogenerated excitons in WS2 are split across the heterostructure by electron transfer from WS2 to graphene. Kelvin probe microscopy showed that regions of graphene covered with WS2 had a smaller work function and supports the model of electron transfer from WS2 to graphene. Our results demonstrate the formation of junctions within a graphene transistor through the spatial tuning of the work function of graphene using these 2D vertical heterostructures.
View details for DOI 10.1021/acsami.5b08295
View details for Web of Science ID 000369044100014
View details for PubMedID 26756350
Electroluminescence Dynamics across Grain Boundary Regions of Monolayer Tungsten Disulfide
2016; 10 (1): 1093-1100
We study how grain boundaries (GB) in chemical vapor deposition (CVD) grown monolayer WS2 influence the electroluminescence (EL) behavior in lateral source-drain devices under bias. Real time imaging of the WS2 EL shows arcing between the electrodes when probing across a GB, which then localizes at the GB region as it erodes under high bias conditions. In contrast, single crystal WS2 domains showed no signs of arcing or localized EL. Analysis of the eroded GB region shows the formation of micro- and nanoribbons across the monolayer WS2 domains. Comparison of the EL spectrum with the photoluminescence spectrum from the monolayer WS2 shows close agreement, indicating the EL emission comes from direct band gap excitonic recombination. These results provide important insights into EL devices that utilize CVD grown monolayer transition metal dichalcogenides when GBs are present in the active device region.
View details for DOI 10.1021/acsnano.5b06408
View details for Web of Science ID 000369115800119
View details for PubMedID 26636982
Mixed multilayered vertical heterostructures utilizing strained monolayer WS2
2016; 8 (5): 2639-2647
Creating alternating layers of 2D materials forms vertical heterostructures with diverse electronic and opto-electronic properties. Monolayer WS2 grown by chemical vapour deposition can have inherent strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS2 and can be exploited in a wide range of applications. We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS2, with Boron Nitride and Graphene. The 2D materials are all grown by CVD, enabling large area vertical heterostructures to be formed. WS2 monolayers grown by CVD directly on Si substrates with SiO2 surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS2. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.
View details for DOI 10.1039/c5nr06770g
View details for Web of Science ID 000369591400019
View details for PubMedID 26758782
Uniformity of large-area bilayer graphene grown by chemical vapor deposition
2015; 26 (39)
Graphene grown by chemical vapor deposition (CVD) on copper foils is a viable method for large area films for transparent conducting electrode (TCE) applications. We examine the spatial uniformity of large area films on the centimeter scale when transferred onto both Si substrates with 300 nm oxide and flexible transparent polyethylene terephthalate substrates. A difference in the quality of graphene, as measured by the sheet resistance and transparency, is found for the areas at the edges of large sheets that depends on the supporting boat used for the CVD growth. Bilayer graphene is grown with uniform properties on the centimeter scale when a flat support is used for CVD growth. The flat support provides consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights into the upscaling of CVD methods for growing high quality graphene and its transfer onto flexible substrates for potential applications as a TCE.
View details for DOI 10.1088/0957-4484/26/39/395601
View details for Web of Science ID 000361013200007
View details for PubMedID 26349521
Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging
2015; 9 (4): 3695-3703
Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.
View details for DOI 10.1021/acsnano.5b00852
View details for Web of Science ID 000353867000031
View details for PubMedID 25870912
Layer-Dependent Modulation of Tungsten Disulfide Photoluminescence by Lateral Electric Fields
2015; 9 (3): 2740-2748
Large single-crystal domains of WS2 are grown by chemical vapor deposition, and their photoluminescent properties under a lateral electric field are studied. We demonstrate that monolayer and bilayer WS2 have opposite responses to lateral electric fields, with WS2 photoluminescence (PL) substantially reduced in monolayer and increased in bilayers with increasing lateral electric field strength. Temperature-dependent PL measurements are also undertaken and show behavior distinctly different than that of the lateral electric field effects, ruling out heating as the cause of the PL changes. The PL variation in both monolayer and bilayer WS2 is attributed to the transfer of photoexcited electrons from one conduction band extremum to another, modifying the resultant recombination pathways. This effect is observed in 2D transition metal dichalcogenides due to their large exciton binding energy and small energy difference between the two conduction band extrema.
View details for DOI 10.1021/nn506594a
View details for Web of Science ID 000351791800050
View details for PubMedID 25706441
Wired Up: Interconnecting Two-Dimensional Materials with One-Dimensional Atomic Chains
2014; 8 (12): 11907-11912
Atomic wires are chains of atoms sequentially bonded together and epitomize the structural form of a one-dimensional (1D) material. In graphene, they form as interconnects between regions when the nanoconstriction eventually becomes so narrow that it is reduced to one atom thick. In this issue of ACS Nano, Cretu et al. extend the discovery of 1D atomic wire interconnects in two-dimensional (2D) materials to hexagonal boron nitride. We highlight recent progress in the area of 1D atomic wires within 2D materials, with a focus on their atomic-level structural analysis using aberration-corrected transmission electron microscopy. We extend this discussion to the formation of nanowires in transition metal dichalcogenides under similar electron-beam irradiation conditions. The future outlook for atomic wires is considered in the context of new 2D materials and hybrids of C, B, and N.
View details for DOI 10.1021/nn5065524
View details for Web of Science ID 000347138000002
View details for PubMedID 25474120
- Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition CHEMISTRY OF MATERIALS 2014; 26 (22): 6371-6379
- Controlling sulphur precursor addition for large single crystal domains of WS2 NANOSCALE 2014; 6 (20): 12096-12103