Research 

Research interests

Highlights

Isotope engineering in 2D Materials to understand the quasiparticle interactions in 2D composites: i) Carbon isotope engineering in monolayer graphene 

Due to the extraordinary mass sensitivity of phonon-energy in graphene, isotopically-labeled layers provide a unique platform for probing physicochemical properties of chemically-tailored-graphene in nanoscale, which otherwise remains a great challenge. This study on 12C and 13C isotope-based fluorinated bilayer graphene unveils fluorination sites, selectivity and stability, providing a way to produce tailored-graphene for high-performance nano-optoelectronic devices.


ii) Sulfur isotope engineering of exciton and lattice dynamics in MoS2 monolayers

The optoelectronic properties of two-dimensional (2D) atomically thin transition metal dichalcogenides (TMDCs) are predominantly governed by excitons and their interaction with lattice and various physical fields. Therefore, it is essential to understand the role played by excitons in the light–matter interaction processes. We introduce sulfur isotope engineering for the first time in the TMDC family, which enables us to disentangle the crucial role played by phonons in the optoelectronic properties of TMDCs. 



Mediator-assisted synthesis of WS2 with ultrahigh-optoelectronic performance at multi-wafer scale

The integration of 2D materials into future applications relies on advances in their quality and production. We here report a synthesis method that achieves ultrahigh optoelectronic performance at unprecedented fabrication scales. A mediator-assisted chemical vapor deposition process yields tungsten-disulfide (WS2) with near-unity photoluminescence quantum yield, superior photosensitivity and improved environmental stability. This enhancement is due to the decrease in the density of lattice defects and charge traps brought about by the self-regulating nature of the growth process. This robustness in the presence of precursor variability enables the high-throughput growth in atomically confined stacks and achieves uniform synthesis of single-layer WS2 on dozens of closely packed wafers. Our approach enhances the scientific and commercial potential of 2D materials as demonstrated in producing large-scale arrays of record-breaking optoelectronic devices.

Superradiant Emission from Coherent Excitons in van Der Waals Heterostructures

This report demonstrates that strong suppression of phonon population at low-temperature results in a formation of a coherent excitonic-dipoles ensemble in the heterostructure, and the collective oscillation of those dipoles stimulates a robust phase synchronized ultra-narrow band superradiant emission even at extremely low pumping intensity. Such emitters are in high demand for a multitude of applications, including fundamental research on many-body correlations and other state-of-the-art technologies.

Through advanced experimental techniques on CrI3 single crystals, we derive a pressure-temperature phase diagram. We find that Tc increases to ∼ 66 K with pressure up to ∼3 GPa followed by a decrease to ∼10 K at 21.2 GPa. The experimental results are reproduced by theoretical calculations based on density functional theory where electron-electron interactions are treated by a static on-site Hubbard U on Cr 3d orbitals. The origin of the pressure-induced reduction of the ordering temperature is associated with a decrease in the calculated bond angle, from 95∘ at ambient pressure to ∼85∘ at 25 GPa. Above 22 GPa, experiment and theory jointly point to the idea that the ferromagnetically ordered state is destroyed, giving rise first to a complex, unknown magnetic configuration, and at sufficiently high pressures a pure antiferromagnetic configuration. This sequence of transitions in the magnetism is accompanied by a well-detected pressure-induced semiconductor-to-metal phase transition that is revealed by both high-pressure resistivity measurements and ab initio theory.

Here we demonstrate a simple design and highly efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core–shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multicolor lasing emitted from core–shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables decreasing the energy consumption of photon propagation. As a result, the energy upconversion emission is enhanced by ∼50 times with a drastic reduction of the lasing threshold. The multiple scatterings arising from the inherent nature of the disordered nanoparticle matrix provide a convenient way for the formation of closed feedback loops, which is beneficial for the coherent laser action.

2D materials with wrinkled structures exhibit superior advantages of high stretchability and a suitable matrix for photon trapping in between the hill and valley geometries compared to their flat counterparts. Here, the intriguing functionalities of wrinkled reduced graphene oxide, single-layer graphene, and few-layer hexagonal boron nitride, respectively, are utilized to design highly stretchable and wearable random laser devices with ultralow threshold. Using methyl-ammonium lead bromide perovskite nanocrystals (PNC) to illustrate the working principle, the lasing threshold is found to be ≈10 µJ cm−2, about two times less than the lowest value ever reported. In addition to PNC, it is demonstrated that the output lasing wavelength can be tuned using different active materials such as semiconductor quantum dots.

We demonstrate a simple architecture composed of graphene quantum dots sandwiched by graphene layers can exhibit several intriguing features, including the Dirac point induced ultralow-threshold laser, giant peak-to-valley ratio (PVR) with ultra-narrow spectra of negative differential resistance and quantum oscillations of current as well as light emission intensity. In particular, the threshold of only 12.4 nA cm−2 is the lowest value ever reported on electrically driven lasers, and the PVR value of more than 100 also sets the highest record compared with all available reports on graphene-based devices. We show that all these intriguing phenomena can be interpreted based on the unique band structures of graphene quantum dots and graphene as well as resonant quantum tunneling.

We have designed and demonstrated a direct WLED consisting of a strontium-based metal–organic framework (MOF), {[Sr(ntca)(H2O)2]·H2O}n (1), graphene, and inorganic semiconductors, which can generate a bright white light emission. In addition to the suitable design of a MOF structure, the demonstration of electrically driven white light emission based on a MOF is made possible by the combination of several factors including the unique properties of graphene and the appropriate band alignment between the MOF and semiconductor layer. Because electroluminescence using a MOF as an active material is very rare and intriguing and a direct WLED is also not commonly seen, our work here therefore represents a major discovery which should be very useful and timely for the development of solid-state lighting.

A composite graphene and graphene quantum dot (GQD) photodetector on lead zirconate titanate (Pb(Zr0.2Ti0.8)O3) (PZT) substrates has been designed to form an ultrasensitive photodetector over a wide range of illumination power. Under 325 nm UV light illumination, the device shows sensitivity as high as 4.06 × 109 A W−1, which is 120 times higher than reported sensitivity of the same class of devices. Plant derived GQD has a broad range of absorptivity and is an excellent candidate for harvesting photons generating electron–hole pairs. Intrinsic electric field from PZT substrate separates photogenerated electron–hole pairs as well as provides the built-in electric field that causes the holes to transfer to the underlying graphene channel.

With the rapid development of technology, electronic devices have become omnipresent in our daily life as they have brought much convenience in every aspect of human activity. Side-by-side, electronic waste (e-waste) has become a global environmental burden creating an ever-growing ecological problem. The transient device technology in which the devices can physically disappear completely in different environmental conditions has attracted widespread attention in recent years owing to its emerging application potential spanning from biomedical to military use. In this work, we demonstrated the first attempt for a dissolvable ecofriendly flexible photodetector using a hybrid of graphene and chlorophyll on a poly(vinyl alcohol) substrate. The whole device can physically disappear in aqueous solutions in a time span of ∼30 min, while it shows a photoresponsivity of ∼200 A W–1 under ambient conditions. The high carrier mobility of graphene and strong absorption strength of a green photon harvesting layer, chlorophyll, result in the photocurrent gain of the device as high as 103 with subsecond response time under the illumination of red light.