Charge and Spin Transport Studies in Graphene and Black Phosphorus, Gavin Koon Kok Wai, PhD Thesis, Department of Physics, National University of Singapore, Barbaros Özyilmaz, Advisor (2015)
Transport studies in graphene and black phosphorus two-dimensional systems will be explored in this thesis. Specifically, I studied the spin transport and spin characteristics of graphene subjected to an enhancement of its otherwise low intrinsic spin-orbit coupling. Taking advantage of its flexibility for engineering modification, we enhanced the spin-orbit coupling via chemical functionalization and metallic adatom decoration. With the initial aim of studying spin transport in black phosphorus which has an energy band gap, I unexpectedly uncovered black phosphorus’ potential as an outstanding thermoelectric material. Our discovery also agrees well with a recent theoretical prediction of high thermopower factor in black phosphorus. The published works on graphene spintronics described in this thesis are both scientifically enlightening and technologically promising. We have also demonstrated the first thermoelectric response in few layer black phosphorus crystals and the performance of this elemental semiconductor is comparable to the state of the art hybrid heterostructures/nanostructures.
See also: http://pubs.acs.org/doi/abs/10.1021/acsnano.5b01922
Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus, Jing Wu, Gavin Kok Wai Koon, Du Xiang, Cheng Han, Chee Tat Toh, Eeshan S. Kulkarni, Ivan Verzhbitskiy, Alexandra Carvalho, Aleksandr S. Rodin, Steven P. Koenig, Goki Eda, Wei Chen, A. H. Castro Neto and Barbaros Özyilmaz, ACS Nano (24 July 2015), DOI:10.1021/acsnano.5b01922
Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼ 3 × 1013 Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼ 9 × 104 A W–1 by applying a source-drain bias (VSD) of 3 V, which is the highest ever measured in any 2D material and 107 times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature.
Also see also: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00775
Large Frequency Change with Thickness in Interlayer Breathing Mode—Significant Interlayer Interactions in Few Layer Black Phosphorus, Xin Luo, Xin Lu, Gavin Kok Wai Koon, Antonio H. Castro Neto, Barbaros Özyilmaz, Qihua Xiong and Su Ying Quek, Nano Lett., 15 (6), 3931–3938 (11 May 2015), DOI:10.1021/acs.nanolett.5b00775
Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.
And finally, for background:
Advances in thermoelectrics: From single phases to hierarchical nanostructures and back, Mercouri G. Kanatzidis, MRS Bulletin, 40, 8, 687-695 (August 2015), Published online by Cambridge University Press, DOI:10.1557/mrs.2015.173
With more than two-thirds of utilized energy being lost as waste heat, there is compelling motivation for high-performance thermoelectric materials that can directly convert heat to electrical energy. However, over the decades, practical realization of thermoelectric materials has been limited by the hitherto low figure of merit, ZT, which governs the Carnot efficiency. This article describes our long-standing efforts to advance ZT to record levels starting from exploratory synthesis and evolving into the nanostructuring and panoscopic paradigm, which has helped to usher in a new era of investigation for thermoelectrics. The term panoscopic is meant as an attempt to integrate all length scales and multiple physical concepts into a single material. As in any other energy-conversion technology involving materials, thermoelectrics research is a challenging exercise in taming “contra-indicated” properties. Critical properties such as high electrical conductivity, thermoelectric power, low thermal conductivity, and mechanical strength do not tend to favor coexistence in a single material. How these can be achieved in certain systems leading to record values of ZT is also described. Endotaxial nanostructures and mesoscale engineering in thermoelectrics enable effective phonon scattering with negligible electron scattering. By combining all relevant length scales hierarchically, we can achieve large enhancements in thermoelectric performance. The field, however, continues to produce surprises.
Footnote: The following article is based on the MRS Medal presentation given by Mercouri G. Kanatzidis at the 2014 Materials Research Society Fall Meeting in Boston. Kanatzidis was recognized “For the discovery and development of nanostructured thermoelectric materials.”
Ok, time to move on this.