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Towards ideal electrical contacts for 2D semiconductors

The contact between metals and semiconductors is the foundation of modern-day electronics. The dimensions of electronics have decreased to the point where bulk semiconductor-based transistors experience high off state currents – leading to heat and power dissipation in devices. Two-dimensional (2D) semiconductors that are atomically thin can mitigate these concerns in short channel transistors. However, to reap the benefits of 2D semiconductors, contact resistance must be reduced down to the quantum limit. 

Our group achieved a breakthrough in 2014 by using phase engineering to realize low contact resistance contacts on few-layered MoS2 (Nature Materials 13, 1128, 2014).


Recently, we have demonstrated controlled creation of ultra-clean van der Waals electrical contacts between three-dimensional (3D) indium metal and atomically thin two-dimensional (2D) semiconductors. Unlike other metals that damage the atomically thin semiconductors due to kinetic energy transfer and/or chemical reactions, we have discovered the gentle low temperature deposition of soft indium metal by thermal evaporation leads to pristine undamaged interfaces (See Figure below and Nature 568, 70 – 74, 2019). Such ultra-clean metal-semiconductor interfaces are one essential parameter for realizing high performance electronics with 2D semiconductors.



Metallic 2D Materials for Electrocatalytic Reduction of CO2 and N2 




Metallic 2D materials have recently emerged as a class of potentially inexpensive catalysts with enhanced activity and increased stability. However, their overall activity relative to state-of-the-art noble metal catalysts is limited by slower reaction kinetics and high charge transfer resistance. Our group’s seminal work several years ago (Nature Materials 12, 850-855, 2013) revealed that these factors can be mitigated by metallic 2D materials. In subsequent works (Nature Materials 15, 1003 - 1009, 2016), we have shown that increasing the metallic phase concentration and lowering the electrical resistance between the support and catalyst nanoparticles can lead to substantial improvement in catalytic performance. An example of this is shown in the Figure (Nature Materials, 2019), which shows that with metallic NbS2 it is possible to achieve very high current densities (several Amps-cm-2) in proof of concept electrolyzer devices for electrocatalytically generating hydrogen. We expect metallic nanosheets to be exceptionally good catalysts for the HER or CO2RR and other reactions. We are therefore investigating the properties of 2D materials towards CO2 reduction and the hydrogen evolution reactions to identify the best candidates for both reactions.






Microelectrochemical cells for catalysis and energy storage

Our group has developed microelectrochemical cells (See Figure and Nature Materials 15, 1003 - 1009, 2016) that allow precise identification of catalytically active sites. We can lithographically pattern devices so that only specific regions of the catalyst material (e.g. edge or basal plane) with well-defined number of atoms are exposed to the electrolyte. Thus, we can accurately quantify the activity of edge sites or the basal plane and because we know the exact area of the patterned electrocatalytically active region, it is possible to precisely extract fundamental parameters such as the exchange current density, the turnover frequency, and Tafel slopes for atomic active sites.


Microelectrochemical cells are also ideally suited for in-situ study of structural changes as well as quantum transport during electrochemical cycling. We fabricate nanodevices to drive intercalation into electrodes from chemically exfoliated and mechanical exfoliated nanosheets. We make vertical heterostructures of different 2D materials to study intercalation dynamics at different interfaces. Multiple metal contacts are deposited to form Hall bars so that electrical properties such as sheet resistance, carrier density from Hall voltage and Hall mobility of the carriers can be measured during electrochemical charging and discharging.


Interfaces between 2D materials and complex oxides

In this project, we integrate complex oxides 2D transition metal dichalcogenides (TMDs) to create interfaces exhibiting exotic phenomena to drive advances in quantum electronics and optoelectronics. The coupling of various degrees of freedom leads to exotic optical and electronic states of matter. Investigation and study of such phenomena is the central theme of modern solid-state physics. Many-body interactions occur at interfaces between heterogeneous materials where new phases of matter can appear. The classic example of this is the LaAlO3/SrTiO3 (LAO/STO) interface that exhibits fundamentally new and different properties from the individual constituents.

Given the unique properties of oxides and 2D materials, their interfaces are expected to reveal intriguing and exotic optical and electronic phenomena. In particular, transition metal d-orbitals tend to hybridize with oxygen/sulfur p-orbitals. This usually results in a mixed p-d electronic state. It is expected that interfacial hybridization effects should also play an important role at the 2D material/oxide heterointerface. We have recently demonstrated (see Advanced Science, to appear) that integration of 2D MoS2 on STO leads to unique electron-phonon coupling that gives rise to a new excitonic state at 3.8 eV – well above the band gap of monolayer MoS2 (1.8 eV). This new high energy exciton is not present in pure MoS2 or STO and is attributed to interfacial orbital-hybridization that couples electron-hole interactions, while the Fermi-surface is modified by interfacial charge-transfer and hybridization.