Results and Discussions Analysis of the Raman spectrum of graphene measured at different temperatures (Fig. 2) reveals that when the temperature increases, the G peak frequency of graphene decreases. By contrast, when the temperature decreases, the G peak frequency increases. The 2D peak shows the same trend and widens when the temperature increases. The G peak frequency (Fig. 4) of the few-layer suspended graphene also differs under different bias voltages. With an increase in the bias voltage, the Joule heat of graphene increases, the temperature of suspended graphene increases, the frequency of the G peak decreases, and a red shift occurs. Consider a situation in which the laser power remains unchanged while the applied bias voltage is altered. Combined with the temperature coefficient of the G peak, the changes in the Raman spectrum at the central position of the suspended graphene under different voltages can be extracted. This allows us to determine the relationship between the G peak frequency of the suspended graphene under different applied bias voltages. The temperature at the central position of the suspended graphene under different bias voltages can then be calculated.

With the rapid development of semiconductor preparation technology and the discovery of new two-dimensional materials, the sizes of semiconductor devices continue to shrink to the micro-nano scale, and devices are being increasingly integrated. However, the heat generation of devices isconcentrated in a smaller range, and the thermal energy density is doubled. The problem of heat dissipation restricts the further reduction in size of nanodevices, and the thermal management in nanodevices cannot be ignored. As a major thermal property of materials, thermal conductivity is critical in optimizing the thermal management of nanodevices. For example, for block materials, the 3ω method is generally used; whereas for nanomaterials, Raman spectroscopy, the suspended thermal bridge method, and the time-domain thermal reflection method are often used. As a typical low-dimensional material, graphene has a high thermal conductivity, unique electron-phonon interaction mechanism, and potential application in the field of thermal management at micro and nano scales. Graphene has thus attracted considerable attention from many researchers. Several studies have been conducted on the thermal conductivity of graphene at different temperatures using theoretical and experimental methods. However, accurate measurements of the thermal conductivity of electrically biased suspended graphene are extremely challenging. In this study, the thermal conductivity of few-layer suspended graphene at different voltages is investigated using Raman spectroscopy.

The device structure is shown in Fig. 1(a). The manufacturing process can be divided into three steps: 1) electrode production; 2) channel etching; 3) graphene transfer. The temperature of graphene changes with the bias voltage. In our study, the bias voltage was fixed, the laser power was changed, and the Raman spectrum of graphene was measured, with the absorption of the laser by graphene causing a local temperature rise and a change in the lattice structure of graphene. The temperature at the center of the device can be estimated from the frequency shift of the G or 2D peak. In our experiment, the laser was aimed at the center of the suspended graphene, where the laser wavelength was 532 nm, and a low laser power (0.5 mW) was used to avoid the thermal effect of the laser. Raman spectroscopy can be used to measure the thermal conductivity of few-layer suspended graphene in two steps: 1) vary the environmental temperature, determine the relationship between temperature and the Raman spectrum (Fig. 2), and obtain the first-order temperature coefficient; 2) fix the bias voltage, change the laser power, determine the relationship between the laser power and characteristic peak frequency (Fig. 4), and calculate the thermal conductivity using the heat flow equation.

Based on Raman spectroscopy, we study the changes in the thermal conductivity of few-layer suspended graphene under different bias voltages. The experimental study shows that when the bias voltage increases from 0 V to 1.5 V, the temperature range is 300-779 K, and the thermal conductivity changes correspondingly, ranging from 2390 to 3000 W/(m·K). This study provides a reference for the study of the heat conduction characteristics of suspended graphene in practical applications.