Staff directory Peng Xiao

Peng Xiao

Postdoctoral Researcher
Phononic and Photonic Nanostructures



  • Controlling the electrochemical hydrogen generation and storage in graphene oxide by in-situ Raman spectroscopy

    Pinilla-Sánchez A., Chávez-Angel E., Murcia-López S., Carretero N.M., Palardonio S.M., Xiao P., Rueda-García D., Sotomayor Torres C.M., Gómez-Romero P., Martorell J., Ros C. Carbon; 200: 227 - 235. 2022. 10.1016/j.carbon.2022.08.055.

    Hydrogen, generated from water splitting, is postulated as one of the most promising alternatives to fossil fuels. In this context, direct hydrogen generation by electrolysis and fixation to graphene oxide in an aqueous suspension could overcome storage and distribution problems of gaseous hydrogen. This study presents time-resolved determination of the electrochemical hydrogenation of GO by in-situ Raman spectroscopy, simultaneous to original functional groups elimination. Hydrogenation is found favoured by dynamic modulation of the electrochemical environment compared to fixed applied potentials, with a 160% increase of C–H bond formation. Epoxide groups suppression and generated hydroxide groups point at these epoxide groups being one of the key sites where hydrogenation was possible. FTIR revealed characteristic symmetric and asymmetric stretching vibrations of C–H bonds in CH2 and CH3 groups. This shows that hydrogenation is significantly also occurring in defective sites and edges of the graphene basal plane, rather than H-Csp3 groups as graphane. We also determined a −0.05 VRHE reduction starting potential in alkaline electrolytes and a 150 mV cathodic delay in acid electrolytes. The identified key parameters role, together with observed diverse C-Hx groups formation, points at future research directions for large-scale hydrogen storage in graphene. © 2022

  • Effect of crystallinity and thickness on thermal transport in layered PtSe2

    El Sachat A., Xiao P., Donadio D., Bonell F., Sledzinska M., Marty A., Vergnaud C., Boukari H., Jamet M., Arregui G., Chen Z., Alzina F., Sotomayor Torres C.M., Chavez-Angel E. npj 2D Materials and Applications; 6 (1, 32) 2022. 10.1038/s41699-022-00311-x.

    We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe2 films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe2 films (~30%). Moreover, we show strong interlayer interactions in PtSe2, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms−1 in bilayer to 1873 ms−1 in eight layers of PtSe2. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics. © 2022, The Author(s).


  • Anisotropic Thermal Conductivity of Crystalline Layered SnSe2

    Xiao P., Chavez-Angel E., Chaitoglou S., Sledzinska M., Dimoulas A., Sotomayor Torres C.M., El Sachat A. Nano Letters; 21 (21): 9172 - 9179. 2021. 10.1021/acs.nanolett.1c03018. IF: 11.189

    The degree of thermal anisotropy affects critically key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe2 films of varying thickness (16-190 nm) and uncover a thickness-independent thermal conductivity anisotropy ratio of about ∼8.4. Experimental data obtained using Raman thermometry and frequency domain thermoreflectance showed that the in-plane and cross-plane thermal conductivities monotonically decrease by a factor of 2.5 with decreasing film thickness compared to the bulk values. Moreover, we find that the temperature-dependence of the in-plane component gradually decreases as the film becomes thinner, and in the range from 300 to 473 K it drops by more than a factor of 2. Using the mean free path reconstruction method, we found that phonons with MFP ranging from ∼1 to 53 and from 1 to 30 nm contribute to 50% of the total in-plane and cross-plane thermal conductivity, respectively. © 2021 The Authors. Published by American Chemical Society.

  • Optomechanical crystals for spatial sensing of submicron sized particles

    Navarro-Urrios D., Kang E., Xiao P., Colombano M.F., Arregui G., Graczykowski B., Capuj N.E., Sledzinska M., Sotomayor-Torres C.M., Fytas G. Scientific Reports; 11 (1, 7829) 2021. 10.1038/s41598-021-87558-4. IF: 4.380

    Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria. © 2021, The Author(s).

  • Reversing the Humidity Response of MoS2- And WS2-Based Sensors Using Transition-Metal Salts

    Xiao P., Mencarelli D., Chavez-Angel E., Joseph C.H., Cataldo A., Pierantoni L., Sotomayor Torres C.M., Sledzinska M. ACS Applied Materials and Interfaces; 13 (19): 23201 - 23209. 2021. 10.1021/acsami.1c03691. IF: 9.229

    Two-dimensional materials, such as transition-metal dichalcogenides (TMDs), are attractive candidates for sensing applications due to their high surface-to-volume ratio, chemically active edges, and good electrical properties. However, their electrical response to humidity is still under debate and experimental reports remain inconclusive. For instance, in different studies, the impedance of MoS2-based sensors has been found to either decrease or increase with increasing humidity, compromising the use of MoS2 for humidity sensing. In this work, we focus on understanding the interaction between water and TMDs. We fabricated and studied humidity sensors based on MoS2 and WS2 coated with copper chloride and silver nitrate. The devices exhibited high chemical stability and excellent humidity sensing performance in relative humidity between 4 and 80%, with response and recovery times of 2 and 40 s, respectively. We have systematically investigated the humidity response of the materials as a function of the type and amount of induced metal salt and observed the reverse action of sensing mechanisms. This phenomenon is explained based on a detailed structural analysis of the samples considering the Grotthuss mechanism in the presence of charge trapping, which was represented by an appropriate lumped-element model. Our findings open up a possibility to tune the electrical response in a facile manner and without compromising the high performance of the sensor. They offer an insight into the time-dependent performance and aging of the TMD-based sensing devices. © 2021 American Chemical Society.