#monolayers

New Self-Assembled Monolayer Is Resistant to Air – Buckyballs Can Pave the Way for Molecular Electronics

Organic self-assembled monolayers (SAMs) have been around for over forty years. The most widely used form is based on thiols, bound to a metal surface. However, although the thiol SAMs are very versatile, they are also chemically unstable. Exposure of these monolayers to air will lead to oxidation and breakdown within a single day. University of Groningen scientists have now created SAMs using buckyballs functionalized with ‘tails’ of ethylene glycol. These molecules produce self-assembled monolayers that have all the properties of thiol SAMs but remain chemically unchanged for several weeks when exposed to air. This robustness makes them much easier to use in research and in devices. An article about these new SAMs was published in Nature Materials recently.

Self-assembled monolayers are dynamic structures, explains University of Groningen Associate Professor of Organic-Materials Chemistry and Devices Ryan Chiechi: ‘These monolayers self-repair and the molecules will continually find the most efficient packing. Furthermore, all processes are reversible, and it is possible to change their composition.’ This distinguishes SAMs from other monolayers that are used to functionalize surfaces. ‘These are often very stable, but they don’t self-assemble and lack the dynamics of SAMs.’

Synthesis of pure single layer of blue phosphorus could be useful for semiconduction

NUS chemists have developed a method to synthesize monolayer blue phosphorus for potential semiconductor applications.

The ability to design and produce materials synthetically could complement the library of naturally occurring layered solids and provide opportunities to develop new materials with unique properties suited for specialized applications. The class of two-dimensional (2-D) elemental materials is particularly interesting as they represent the chemically simplest case for materials development, and serve as a model system for exploring on-surface synthesis mechanisms. One such 2-D materials is blue phosphorus, which has been calculated to possess a large band gap suitable for use in optoelectronic devices.

A research team led by Prof Chen Wei from the Department of Chemistry and Department of Physics, NUS has demonstrated experimentally that an atomically-thin layer of pure blue phosphorus can be obtained from silicon intercalation of a blue phosphorus-gold (BlueP-Au) alloy. A single layer of BlueP-Au alloy was prepared by depositing bulk black phosphorus precursors onto a heated gold (111) surface. Silicon molecules were subsequently evaporated from a heated silicon rod. These silicon atoms inserted themselves below the BlueP-Au alloy and formed a gold silicide buffer layer, breaking the molecular bonds between the phosphorus and gold atoms, thereby leading to the formation of a pure single layer of blue phosphorus on the surface.

Gold nanoparticles enhance light emissions from tungsten disulphide

NUS physicists have discovered that gold nanoparticles can enhance light emissions from tungsten disulphide (WS2) flakes and reveal minute changes in the material composition.

Two-dimensional (2-D) transition metal dichalcogenides (TMDs) show great potential as sensors and optoelectronic devices as they are able to exhibit strong optical (fluorescent) signals. Tungsten disulfide (WS2), a type of TMD, has strong optical properties that are sensitive to its structural and chemical composition. Methods to functionalise its fluorescent properties are of interest because if the sensing action results in a colour change in the material, it becomes easier for the user to detect it. This fluorescence is due to the recombination of electron-hole pairs in the 2-D TMDs, which also have an associated electrical response that can be used for possible optoelectronic applications.

A research team led by Prof SOW Chorng Haur, from the Department of Physics, NUS has discovered that the addition of gold nanoparticles (Au NPs) to WS2 strengthens the fluorescence emissions from it, with the higher light intensity dominated by excitons. The Au NPs also behave like nano-explorers, exhibiting preferential, site-selective decoration that maps out interesting fluorescent patterns within the WS2 monolayers! These patterns are totally unexpected as the researchers had initially thought that the Au NPs would spread themselves in a random manner across the material.

Side-by-side deposition of atomically flat semiconductor sheets enhances solar cell conversion efficiency

Super thin photovoltaic devices underpin solar technology and gains in the efficiency of their production are therefore keenly sought. KAUST researchers have combined and rearranged different semiconductors to create so-called lateral p-n heterojunctions—a simpler process they hope will transform the fabrication of solar cells, self-powered nanoelectronics as well as ultrathin, transparent, flexible devices.

Two-dimensional semiconductor monolayers, such as graphene and transition-metal dichalcogenides like WSe2 and MoS2, have unique electrical and optical properties that make them potential alternatives to conventional silicon-based materials. Recent advances in material growth and transfer techniques have allowed scientists to manipulate these monolayers. Specifically, vertical stacking has led to ultrathin photovoltaic devices but requires multiple complex transfer steps. These steps are hampered by various issues, such as the formation of contaminants and defects at the monolayer interface, which limit device quality.

"Devices obtained using these transfer techniques are usually unstable and vary from sample to sample," says lead researcher and former visiting student of Associate Professor, Jr-Hau He, Meng-Lin Tsai, who adds that transfer-related contaminants significantly affect device reliability. Electronic properties have also proven difficult to control by vertical stacking.

Multitasking monolayers

Two-dimensional materials that can multitask.

That is the result of a new process that naturally produces patterned monolayers that can act as a base for creating a wide variety of novel materials with dual optical, magnetic, catalytic or sensing capabilities.

"Patterned materials open up the possibility of having two functionalities in a single material, such as catalyzing a chemical reaction while simultaneously serving as a sensor for a second set of molecules," said Sokrates Pantelides, William and Nancy McMinn Professor of Physics at Vanderbilt University, who coordinated the research with Professor Hong-Jun Gao at the Institute of Physics of the Chinese Academy of Sciences in Beijing. "Of course, you can do such a thing by using two materials side by side, but patterned materials offer a whole range of new options for device designers."

Their achievement is described in a paper titled "Intrinsically patterned two-dimensional materials for selective adsorption of molecules and nanoclusters" published Jun. 12 in the journal Nature Materials.

The thinnest materials that can be produced today have the thickness of a single atom. These materials -- known as two-dimensional materials -- exhibit properties that are very different compared with their bulk three-dimensional counterparts. Until recently, 2D materials were produced and manipulated as films on the surface of some suitable 3D substrate. Working in collaboration with a team from the Leibniz Institute for New Materials, a group of physicists at Saarland University, led by Professor Uwe Hartmann, has for the first time succeeded in characterizing the mechanical properties of free-standing single-atom-thick membranes of graphene. The measurements were performed using scanning tunnelling microscopy (STM). The researchers have published their results in the specialist journal Nanoscale.

Two-dimensional materials have only been known for a few years. In 2010, the scientists André Geim and Konstantin Novoselov were awarded the Nobel Prize in Physics for their research work on the material graphene -- a two-dimensional allotrope of pure carbon. Following that discovery, a number of other 2D materials made from silicon or germanium were produced and characterized. "The special feature of these materials is that they are only one atom thick -- they are practically all surface," explains Professor Uwe Hartmann, an experimental physicist at Saarland University. As a result they possess physical properties that are wholly different to their more conventional three-dimensional relatives.

Self-assembled monolayers of NHCs on gold have ballbot-type motion

Self-assembled monolayers are molecules anchored to a surface. These molecules have been used to advance the fields of molecular sensors and electronics. Monolayers on gold surfaces are typically anchored by a thiol. One alternative to thiols are N-heterocyclic carbenes (NHCs). First isolated in 1991, NHCs have proved an important molecule for practical uses because it can serve as a ligand for transition metals as well as some main group elements. NHCs have also been used for organocatalysis, cross-coupling reactions, and pharmaceuticals.

Research groups from Westfälische Wilhelms-Universität in Münster lead by Professors Harold Fuchs and Frank Glorius, used scanning-tunneling microscopy analyses combined with first principles calculations of representative NHC monolayers on gold surfaces to determine how the monolayers can be both chemically robust and highly mobile. They found that the NHC bound to gold forms an adatom that then traverses the gold surface in a ballbot-type motion. Their report appeared in a recent issue of Nature Chemistry.

NHCs form a particularly strong bond with gold because of the sp2 hybridized lone pair of electrons on the carbon atom (See figure of NHCs above). Studies have shown that NHCs can be used to form particularly stable self-assembled monolayers (SAMs) on gold surfaces and can even be modified after formation of the monolayer, but these studies are difficult to compare with each other because they do not have similar R groups.

2D materials researchers aim ‘beyond graphene’

Joshua Robinson recalls the day in 2006 when he learned of a material that is, for all practical purposes, two-dimensional.

At the time, he was a post-doctoral researcher at the Naval Research Laboratory in Washington, D.C. His advisor, Eric Snow, was raving about graphene, a newly isolated form of carbon. A cousin of the widely known buckminsterfullerene (or “buckyballs”) and carbon…