Prof Andrew Wee’s Surface Science Laboratory at NUS is recognized as one of the internationally leading surface and nanoscale science groups. His research interests include scanning tunneling microscopy (STM) and synchrotron radiation studies of the molecule-substrate interface, organic-inorganic hybrid heterojunctions, 2D materials and devices. Some research highlights from his group in the past decade are:

(1) STM studies of 2D transition metal dichalgogenide (TMD) surfaces and hybrid heterointerfaces (2014-): His group reported the first detailed STM studies on bandgap tunability at single-layer molybdenum disulphide grain boundaries (Nature Comm 2015) and the heterointerface between organic molecules and monolayer TMDs (ACS Nano 2016). They reported the electronic properties of a 1D intrinsic/p-doped heterojunctions in a 2D TMD semiconductor (ACS Nano 2017). They have also recently developed a method to enhance the photoluminescence efficiency of tungsten diselenide monolayers, paving the way for the application of such semiconductors in advanced optoelectronic and photonic devices. This was achieved by incorporating monolayers of tungsten diselenide onto gold substrates with nanosized trenches (plasmonic nanostructures), and successfully enhanced the nanomaterial’s photoluminescence by up to 20,000-fold (Nature Comm 2016). They also studied gap states at low-angle grain boundaries in monolayer tungsten diselenide (Nano Lett 2016), and Oxygen Passivation Mediated Tunability of Trion and Excitons in monolayer MoS2 (PRL 2017).

(2) Phase transformations in 2D TMDs (2016-): More recently, we have been interested in polymorphism of 2D TMDs, such as MoS2 that exhibits fascinating optical and transport properties. We observed a tunable inverted gap (~ 0.50 eV) and a fundamental gap (~ 0.10 eV) in quasimetallic monolayer MoS2 induced by electron doping from gold, facilitated by interfacial tensile strain (Nature Comm 2017). This work is extended to other 2D TMD-metal systems, and we have unraveled the phase‐transition dynamics in TMDs on metallic substrates (Adv. Sci. 2019).

(3) Tuning the surface electronic properties of graphene (2007-): His group was the first to demonstrate surface transfer doping of graphene using non-covalently bonded molecules (JACS 2007), which is important since being a single atomic layer, graphene cannot be easily doped by atom substitution. They also elucidated the bottom-up growth of epitaxial graphene on silicon carbide substrates (ACS Nano 2008), and their STM image was featured in Nature News (25 March 2009). He was invited to present the topic of “Molecular interactions on epitaxial graphene” at the Nobel Symposium on Graphene and Quantum Matter (Saltsjobaden, Sweden, May 2010); this fascinating meeting was where the Nobel committee ‘decided’ on the 2010 Nobel prize in Physics (Physica Scripta 2012). His group also reported novel results on the bottom up (Sci Rep 2012) and top down (Nature Comm 2013) fabrication of armchair graphene nanoribbons (AGNR), and measured their electronic properties and demonstrated prototype diodes and photodetectors based on AGNR-CNT heterojunctions. In 2023, his then RF Dr Guo Jian published a Nature paper on ferroelectricity on single layer bismuth, using our STM/nc-AFM saystem.

(4) Molecular self-assembly on surfaces and organic heterojunctions (2006-): His group demonstrated 0D, 1D and 2D molecular self-assembly on surfaces, and published numerous papers using STM and synchrotron spectroscopic studies. For example, they demonstrated self-assembled organic donor/acceptor nanojunction arrays (APL 2008 cover), and this work was highlighted in Nature Nanotechnology (July 2008). The group is well known for their expertise in assembling tunable 2D binary molecular networks (Small 2010 cover), and for demonstrating reversible single-molecule switching in ordered monolayer molecular dipole arrays (Small 2012). As recognized leaders in this field, his group has published numerous reviews on topics such as single molecule switches (Chem Soc Rev 2015), molecular functionalization of graphene (Prog Surf Sci 2013), organic-organic heterojunction interfaces (Adv Func Mat 2011), surface transfer doping (Prog Surf Sci 2009), and charge transfer across the molecule/metal interface using the core hole clock technique (Surf Sci Rep 2008). This work continues with studies of the organic–2D TMD heterointerface, which has been described in a review in Chem Soc Rev (2018).