Quanmin Guo

Resolving the Au-adatom-alkanethiolate bonding site on Au(111) with domain boundary imaging using high-resolution scanning tunneling microscopy.

The bonding sites for Au-adatom-octanethiolate within the (√3×√3)R30° structure on Au(111) have been investigated with high-resolution scanning tunneling microscopy (STM) imaging. By establishing the relationship between the lateral positions of adsorbates on the top layer of gold and those inside an etch pit, we are able to determine the adsorption configuration with a high degree of accuracy for the elusive (√3×√3)R30° molecular layer. The boundary between adjacent SAM domains is also imaged with molecular resolution that allows the assignment of adsorption site in each domain without ambiguity. The standard (√3×√3)R30° alkanethiol SAM on Au(111) is found to consist of domains with Au-adatom-octanethiolate occupying the fcc hollows site, alongside domains where the hcp hollow site is occupied.

Adsorption Site Determination for Au-Octanethiolate on Au(111)

Self-assembled monolayers (SAMs) of Au-octanethiolate on Au(111) have been studied using scanning tunneling microscopy (STM). Thermal annealing of the dense (square root(3) x square root(3))R30 degrees layer at 353 K for 1 h leads to the formation of a (5 square root(3) x square root(3))R30 degrees striped phase coexisting with the (square root(3) x square root(3))R30 degrees phase. High-resolution STM imaging shows that the unit cell of the (5 square root(3) x square root(3))R30 degrees phase consists of four adsorbed Au-thiolate species giving rise to an adsorbate coverage of 0.27 ML. The four Au-thiolate species take the standing-up orientation and occupy inequivalent adsorption sites: one on a bridge site and three on the hollow sites. By drawing connections between the (5 square root(3) x square root(3))R30 degrees and the (square root(3) x square root(3))R30 degrees phases, it is found that the adsorption site for Au-thiolate inside the (square root(3)3 x square root(3))R30 degrees phase must be either the fcc hollow or the hcp hollow site.

Colloidal Lines and Strings

Self-assembly of polystyrene spheres guided by pattered InP substrates has been studied using atomic force microscopy (AFM). Using photoresist stripes and posts, 1-D and 2-D colloidal particle structures were found to preferentially nucleate at the step edges of the photoresist. The dangling bonds created by photocleavage of molecular bonds at the steps are identified as playing a major role in attracting the colloidal suspension.

Layer by layer removal of Au atoms from passivated Au(111) surfaces using the scanning tunneling microscope: Nanoscale “paint stripping”

Layer by layer removal of gold atoms from the (111) surface of gold has been performed using the scanning tunneling microscope. The process is made possible by a chemisorbed self-assembled monolayer (SAM) of dodecanethiol molecules on the surface, which gives rise to a reduced bonding strength between the top two layers of gold atoms. The gold atoms and associated adsorbed molecules are peeled off and displaced laterally by the STM tip, and the size of the modified area (down to ∼10×10 nm) is more or less determined by the scan size.

Patterned arrays of porous InP from photolithography and electrochemical etching

Patterned arrays of porous InP have been produced using electrochemical etching method at room temperature in combination with photolithography. n-type InP wafers with (001) orientation were used as the anode, and gold was used as the cathode. The porous structure was produced in either aqueous HCl or a mixture of HCl and HNO3 with a voltage bias ranging from 2 to 10 V. Alternating stripes of porous and nonporous InP have been fabricated on an InP substrate by etching a masked sample. Surface morphology measurements and cross sectional analysis of the porous layer have been conducted using atomic force microscopy and scanning electron microscopy. Photoluminescence from the porous surface shows a significant suppression of the interband transition. An energy barrier at the porous/bulk InP interface, identified from conductance measurements, is proposed to arise from the effect of surface states.

Controlling the formation of Au nanoparticles using functionalized molecular buffer layers

Nanoparticles of gold are produced by evaporation of gold atoms onto functionalized self-assembled monolayers (SAMs). SAMs of dodecanethiol (DT) and mercaptoundecanoic acid (MUA) on Au(1 1 1) are used as substrates and the gold particles are imaged with a scanning tunneling microscope. Gold atoms evaporated on DT monolayers diffuse rapidly to the interface between the molecular layer and the gold substrate where they form two-dimensional islands of 1 ML in height. In the case of monolayers of MUA, the strong interaction between evaporated gold and the acid group of the molecule gives rise to effective pinning of the gold atoms on top of the molecular layer and leads to the growth of three-dimensional gold nanocrystals.

Imaging thin films of organic molecules with the scanning tunnelling microscope

This article gives a topical review of the application of the scanning tunnelling microscope (STM) to the characterisation of thin films (typically monolayers) of organic molecules on surfaces. Three classes of molecules are considered: “self-assembled” thiol monolayers, macrocycles, e.g. porphyrins and cyclophanes, and passivated nanoparticles. Typically these molecules are deposited onto the surface of, e.g. gold/graphite, from a solution. The STM is shown to be a tool not just for imaging, from the morphology of the film down to the level of individual molecules, but also for molecular manipulation and for electrical measurements. Indeed, an appropriate understanding of the electrical characteristics of the STM measurement is essential for successful imaging of organic (or biological) molecules.

How Nanoscience Translates into Technology: The Case of Self-Assembled Monolayers, Electron-Beam Writing, and Carbon Nanomembranes

One of the great quests in nanotechnology is to translate nanoprecision materials science into practical manufacturing processes. The paper by Angelova et al. in this issue of ACS Nano, which discusses the production of functional carbon-based membranes with a thickness of 0.5 to 3 nm, provides instructive insight into how researchers are pulling together complementary strands from a quarter century of nanoscience research to develop novel, hybrid processing schemes. In this Perspective, we reflect on the progress that is taking place in the two principal component technologies combined in this scheme, namely, (i) control of self-assembled monolayers, including their detailed atomic structures, and (ii) electron-induced manipulation and processing of molecular layers, as well as considering (iii) remaining challenges for thin membrane production in the future.

The striped phases of ethylthiolate monolayers on the Au(111) surface: A scanning tunneling microscopy study

Striped phases of ethylthiolate monolayers, corresponding to surface coverage in between 0.2 ML and 0.27 ML, were studied using high-resolution scanning tunneling microscopy. Striped phases consist of rows of Au-adatom-diethythiolate (AAD) aligned along the [112] direction. In the perpendicular [110] direction, the AAD rows adjust their spacing according to the surface coverage. A (5√3 × √3)-R30° striped phase with 0.27 ML thiolate and a (6√3 × √3)-R30° striped phase with 0.23 ML thiolate, both with long-range order, are found. A localized (5 × √3)-rect. phase is also found as a minority phase embedded in the 5√3 × √3)-R30° phase. This (5 × √3)-rect. phase can be constructed using di-Au-adatom-tri-thiolate species.

An X-ray photoelectron spectroscopy study of the stability of ZrO2 films on Pd(110)

Thermal stability of ZrO 2 overlayers on a Pd(110) substrate is studied using X-ray photoelectron spectroscopy (XPS). The stoichiometric ZrO 2 films, produced in situ by oxidation of physical vapour deposited Zr metal, are stable in I X 10 -6 Torr of oxygen up to 1000 K. Under vacuum conditions, the oxide films are found to decompose at temperatures above 840 K, resulting in the formation of Zr-Pd alloy. The decomposition of ZrO 2 is found to take place at the oxide-Pd interface and the rate limiting step is the release of oxygen through the boundaries between small oxide crystallites.