Our research has two integrated components: (1) Nanomaterial sysnthesis and nanoscale fabrication and (2) Electrical and optical measurement. Our expertise on material sysnthesis and nanoscale fabrication enables us to design structures/devices which are suitable for our electrical and optical study. We are also constantly developing unique electrical and optical measurement tools which can best characterizing electron-photon interactions in low dimensions. The obtained knowledge, in return, helps us to propose and fabrication novel nanoscale electronic and optoelectronic devices that are promising for future applications.
We use mechanical exfolation of bulk single crystal or chemical vapor deposition to produce atomically thin materials. These high quality materials form the critical foundation for our research. 1) Mechanical exfoliation is a simple yet powerful to produce various two-dimensional (2D) materials from bulk single crystal. It also provides a flexible method to combine different 2D material to form van der Waals heterostructures, through which we can design materials with atomic resolution. 2) Chemical vapor depostion (CVD) provides a way to produce high quality 2D materials over large scale which greatly facilitate our research. More importantly it potential leads to real applications.
We build nanometer scale devices with precise control provided by state-of-art fabrication tools. We also develop fabrication tools in our lab, such as electromigration technique, to go beyond the resolution of ebeam lithography to make devices down to atomic scale. This capability, combined with our nanomaterial synthesis, provides a powerful platform to investigate quantum effects in spatially confined materials. This understanding not only reveals interesting physics in low dimensions but also guides new materials and new devices design. In return, we can implement this understanding with our fabrications capability to create novel electronic or opto-electronic devices for applications.
Dual gated graphene device fabricated using CVD graphene. We fabricate hundreds of devices with high high mobility. This high quality devices enables photocurrent measurement in graphene pn junction, revealing hot carrier behavior in graphene.
Ultra-small plasmon enhanced photo-detector by coupling gold break junction with graphene nanoconstriction (left). The graphene constriction is too small to be seen directly under scanning electron microscope (SEM), and we show it schematically (right).
Single electron transistor (SET) device made by single metallic nanoparticle (Au, Pd, or Pt). We can functionlize the break junction surface and bridge the nanometer scale gap by a chemically synthesized metal nanopartcile. The SEM (left) show a typical device and the differential conductance measurement as a function of gate voltage and bias exhibits typical Coulomb diamond pattern, a result of Coulomb blockade (electrons have to tunnel through one by one).
Electrical and Optical Measurements
We perform low temperature transport measurement to reveal quantum effects in nanoscale devices. We hope to apply the obtained knowledge to design nanoelectronic devices with new applications. We also perform broadband (visible to terahertz) and ultrafast (~100s fs) optical spectroscopy of nanomaterials (mostly 2D materials) to understand light matter interaction in low dimensions. Most importantly, we combine these two and develop spatially resolved and time resolved photo-current measurement to investigate photocurrent generation at nanoscale devices with ultrafast time resolution.
We have used scanning photocurrent setup (schematically shown on the left) to reveal hot carriers response in graphene pn junction (shown on the top right panel). Aided by ultrafast laser pulse, we can also probe the energy relaxation in graphene through photocurrent correlation measurement (right bottom panel). These measurement can be done from room temperature (~ 300 K) to liquid helium temperature (4 K).
Optical pump terahertz probe measurement reveal optical excitation induced graphene conductivity change in ultrafast time scale (~ 1 ps). Using a gated graphene FET device (shown schematically on the left), we also show the unique optical response in graphene dominated by hot carriers.
Low temperature photo-luminescence and absorption measurement (on the right) helps to identify strong exciton behavior in a MoSe2/bilayer graphene heterostructures (STM images shown on the left).
Optical pump probe measurement reveals ultrafast carriers transfer in an atomically thin pn junction.
Magneto-resistance measurement at low temperature reveals quantum interference enhanced anisotropic magneto-resistance (AMR). Left shows a SEM image of the ferromagnetic break junction that we fabricate, while the right shows the differential resistance (dI/dV) of this device measured at 4.2 K as a function of bias and in-plane magnetic field direction.