B.Sc, 1999, University of Groningen, the Netherlands, M.Sc. 2002, University of Groningen, the Netherlands, Ph. D. with distinction cum laude (top 5%), 2006, University of Twente, the Netherlands. Postdoctoral fellow, 2007-2010, Harvard University, USA. Fellow of the National Research Foundation (NRF) of Singapore.
We are interested in organic-inorganic hybrid nanoscale electronic devices. We combine non-conventional fabrication techniques with lithography to produce structures with unique properties to advance fundamental understanding and to improve the performance of devices. Our research mainly focuses on studies of the mechanisms of charge transport at the nano-scale and how this knowledge can be used in molecular electronics, plasmonics, and interface engineering. Our group is located at the Department of Chemistry and the Centre for Advanced 2D Materials (CA2DM).
We design, characterize and study the self-assembly of new molecular structures. We use fabrication techniques such as Photolithography and Electron-Beam Lithography (EBL) available at Graphene Research Centre (GRC) and Institute of Materials Research and Engineering (IMRE) and we characterize our Self Assembled Monolayers (SAMs) with lab-based techniques (FTIR, UV-Vis, ellipsometry etc.) but the facilities at the Singapore Synchrotron Light Source (SSLS) make it possible to record ARXPS, ARUPS, and NEXAFS spectra to obtain detailed information about the electronic and supramolecular structure of the SAMs. See for a full description of our research interests Personal Webpage.
Molecular electronics makes it possible to control the electrical characteristics of a system at the molecular level. Molecular junctions of the form electrode-SAM-electrode are appealing because of their potential for inducing and controlling electronic function at the nanometer scale and SAMs can be readily characterized. We use the eutectic alloy of Gallium and Indium (EGaIn) to form non-invasive contacts to the SAMs. Impedance Spectroscopy (J. Am. Chem. Soc. 2014, 136, 11134-11144) makes it possible to separate the contribution of each component in a molecular junction from each other (e.g., the contact resistance, SAM resistances, SAM capacitance). Recently, we showed that the direction of rectification can be controlled in a molecular diode (Nat. Commun. 2015, 6, 6324) and we developed a molecular diode with the highest rectification ratio to date.
Figure 1: (A) Schematic illustration of the SAM-based junctions with vdW (van der Waals) interface (not drawn to scale) and the equivalent circuit for the junctions (J. Am. Chem. Soc. 2014, 136, 11134-11144; Nanoscale 2015, 7, 12061-12067). (B) Snapshots showing the formation of the tip of GaOx/EGaIn and a tunneling junction (J. Am. Chem. Soc. 2014,136, 1982-1991). (C) The molecular structure of S(CH2)12FcCH3, and the corresponding SAM on silver obtained by molecular dynamics (Nat. Nanotechnol. 2013, 8, 113-118). (D) Molecular Diode with a rectification ratio of three orders of magnitude (Nano Lett. 2015, 15, 5506–5512). (E) The junctions of the form AgTS-SCnFcC13-n//GaOx/EGaIn showing how rectification varies with n (Nat. Commun. 2015, 6, 6324).
We aim to explore the possibilities of self-assembly and supramolecular chemistry in bottom-up nanofabrication to obtain devices which are organized at the molecular level. We use non-classical approaches that are compatible with the relatively fragile molecules to fabricate molecular electronic devices for applications in plasmonics and graphene based electronics.
Figure 2: (A) Schematic illustration of SAM-based tunneling junctions of the form of AgTS−SCn//GaOx/EGaIn along with an optical photograph. EGaIn is stabilized in a microfluidic device made of a transparent rubber of polydimethylsiloxane (PDMS) (Adv. Funct. Mater, 2014, 24, 4442-4456; J. Phys. Chem. C 2015, 119, 960–969). (B) Top to bottom - Schematic of the suspended graphene experiment. Scanning electron micrographs (SEM) of the 100 nm diameter holes in Si3N4, without (L) and with Graphene (R). SEM images after the deposition of 30 nm of Au on the holes without (L) and covered with Graphene (R) (ACS Appl. Mater. Interfaces 2014, 6, 20464−20472). (C) Flowchart of steps involved in Nanoskiving, a method we use to fabricate plasmonic structures. Au microplates deposited on a flat epoxy surface are embedded in a second layer of epoxy from which Au nanobeams are cut using an ultramicrotome. After treatment with air-plasma to remove the epoxy and deposition of a layer of Al2O3, Ag nanowires are drop-casted on the substrate (ACS Photonics 2015, 2, 1348−1354).
To satisfy the increasing need for higher data processing speeds, integrated plasmonic optoelectronic circuits are proposed which have the ability to beat the optical diffraction limit and allow fast information communication at optical frequencies between nanoscale devices. We are developing new platforms to study the interplay between photonics, electronics and plasmonics. For instance, we demonstrated tunneling charge transfer plasmon modes over distances beyond one nanometer (Science, 2014, 343, 1496-1499).
Figure 3: (A) Quantum tunneling between plasmonic resonators (two silver nanoparticles) bridged by a SAM on an electron-transparent Si3N4 membrane. The distance between them is determined by the thickness of the SAMs of EDT or BDT (Science, 2014, 343, 1496-1499). (B) Photoluminescence of a single Au nanobeam (NB) by the surface plasmons of an Ag nanowire (NW) along with the corresponding SEM image of an Ag NW aligned nearly perpendicularly on a single Au NB (ACS Photonics 2015, 2, 1348−1354).
Figure 4: (A) Schematic of the graphene protection barrier for n-Si(111)/Au and n-Si(111)/Cu devices where graphene prevents unwanted doping of the Si by the Au and Cu upon heating (ACS Appl. Mater. Inter. 2014, 6, 20464–20472).
(B) Schematic drawings of the system of recording the vertical current density across two layer-by-layer (LBL)-stacked CVD graphene layers and the experimental data of current density as a function of SAM growth time showing a six order increase of the current with time (Nat. Commun. 2014, 5, Article number: 5461).