Xiaohui Ye

Precise Control of the Number of Layers of Graphene by Picosecond Laser Thinning

The properties of graphene can vary as a function of the number of layers (NOL). Controlling the NOL in large area graphene is still challenging. In this work, we demonstrate a picosecond (ps) laser thinning removal of graphene layers from multi-layered graphene to obtain desired NOL when appropriate pulse threshold energy is adopted. The thinning process is conducted in atmosphere without any coating and it is applicable for graphene films on arbitrary substrates. This method provides many advantages such as one-step process, non-contact operation, substrate and environment-friendly, and patternable, which will enable its potential applications in the manufacturing of graphene-based electronic devices.

Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer.

In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS2 monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS2 monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

Thinning of large-area graphene film from multilayer to bilayer with a low-power CO2 laser

Bilayer graphene has attracted a great deal of attention for many electronic and optical applications. Although large-area bilayer graphene can be synthesized by chemical vapor deposition (CVD), multilayer growth often occurs and subsequent processes are required to obtain uniform bilayer films. We report an efficient way of thinning multilayer graphene film by low-power CO2 laser irradiation in vacuum. With a laser power density of ~10(2) W cm(-2), pristine graphene film of 4-5 layers can be thinned to a bilayer free of defects in 30 s. Contrary to previous laser-assisted graphene thinning processes, which reduced graphene layers precisely and locally with a high power density and a small beam diameter, our approach enables high-efficiency thinning of large-area graphene film whilst using a significantly reduced power density and an increased laser beam diameter.

Direct laser fabrication of large-area graphene: An engineering approach to nano-materials

Graphene is a promising material for many potential applications, owing to its extraordinary properties. Numerous synthesizing approaches have been developed to realize the promising applications of graphene. However, there is lack of a comprehensive way, which combines the following features, i.e. large-area growth, patterned graphene and fast growth at the same time. Here, we provide an engineering approach to both large-area and patterned graphene at a high rate. The high power and continuous-wave lasers were employed to irradiate solid carbon source coated on nickel surface. The rate of graphene production by laser irradiation exceeds 28.8 cm2/min and the solid carbon source is less hazardous compared to typical gaseous carbon sources used in chemical vapor deposition (CVD). The influence of laser process on graphene fabrication was investigated systematically. Raman analyses, optical images and scanning electron microscopy (SEM) results show that the quality of graphene strongly depends on the laser process parameters, including laser power density and scanning rate. Adjusting the parameters to input the appropriate heat, so that the high quality graphene can be obtained in an optimal condition. The influence of carbon source on graphene growth was also investigated. Ultimately, the penetration and precipitation mechanism of carbon into Ni substrate during the fabrication process were also discussed. This approach may reach the scale large enough for practical applications.Graphene is a promising material for many potential applications, owing to its extraordinary properties. Numerous synthesizing approaches have been developed to realize the promising applications of graphene. However, there is lack of a comprehensive way, which combines the following features, i.e. large-area growth, patterned graphene and fast growth at the same time. Here, we provide an engineering approach to both large-area and patterned graphene at a high rate. The high power and continuous-wave lasers were employed to irradiate solid carbon source coated on nickel surface. The rate of graphene production by laser irradiation exceeds 28.8 cm2/min and the solid carbon source is less hazardous compared to typical gaseous carbon sources used in chemical vapor deposition (CVD). The influence of laser process on graphene fabrication was investigated systematically. Raman analyses, optical images and scanning electron microscopy (SEM) results show that the quality of graphene strongly depends on the laser ...