Electrical and Optical Measurement
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.