Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching, angle-resolved XPS (ref. 2) and Tougaard's approach, but all suffer various limitations. Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain 'marker' monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method 'controlled surface charging' and expect it to be generally applicable to a large variety of mesoscopic heterostructures.

A new kind of multilayers based on metal-ion coordination was constructed on gold surfaces, where molecular layers are successively added using a highly controlled step-by-step procedure. A bifunctional ligand is used as the base layer, bearing a cyclic disulfide group to attach to the gold surface and a bishydroxamate group capable of ion binding. An 8-coordinating metal ion such as Zr4+ or Ce4+ is then coordinated to the bishydroxamate site, followed by exposure to a second ligand possessing four hydroxamate groups. The tetrahydroxamate molecule ligates to the metal ion (bound to the base layer) using two of its four hydroxamate groups and is free to bind a second metal ion at its other end. A sequence of adsorption steps using metal ions and tetrahydroxamate ligands was carried out, resulting in an ordered metal−organic multilayer. Multilayer structures comprising up to 10 tetrahydroxamate/metal ion layers were constructed, with full characterization at each step of multilayer formation using ellipsometry, contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The multilayer morphology and mechanical properties were studied by scanning force microscopy. It is shown that different base ligands induce dramatic differences in the morphology and stiffness of the final multilayer. The possibility to construct segmented multilayers containing Zr4+ and Ce4+ ions at defined locations is presented.

Metal-organic coordination is an attractive means for constructing supramolecular systems, providing versatility, simple synthesis, and a defined geometry. The convenience of changing "building blocks" during multilayer assembly is exploited for the fabrication of novel ion-coordinated hybrid multilayers on gold. Two bifunctional linkers are used, a tetrahydroxamate and an organic diphosphonate, while the connection between layers is accomplished through Zr(IV) coordination, to form a well-defined hybrid multilayer. The two ion binders are compatible with respect to multilayer assembly, allowing the change of linkers during construction while maintaining the film structural integrity and organization. The different chemical reactivity of the binders enables rational structural manipulation of the multilayer, by selective dissolution of the acid-sensitive hydroxamate layers while keeping the acid-resistant phosphonates (and underlying hydroxamates) intact. The process demonstrates the multilayer structural quality, where two diphosphonate monolayers are capable of effectively blocking proton penetration to underlying hydroxamate layers. This allows nanometer-scale reshaping of the molecular film according to a scheme introduced during its construction.

A novel type of bilayer on a gold surface, based upon metal-ion coordination to hydroxamate moieties, is described. Tailor-made bifunctional ligands containing hydroxamate groups (for metal coordination) and a cyclic disulfide residue (for surface attachment) have been prepared. The bishydroxamate binding site forms 2:1 ligand/ metal complexes with octacoordinating metal ions such as Zr IV , Ce IV , and Ti IV ; the cyclic disulfide moiety anchors the complex to the gold surface. Two routes to bilayer formation are demonstrated: i) a one-step process from preformed 2:1 complexes, and ii) a stepwise process including formation of the ligand monolayers followed by binding of a guest ion and a second layer of ligand molecules.The former approach allows full characterization of the complexes before bilayer assembly, whereas the latter enables construction of either symmetric (identical) or asymmetric (nonidentical) bilayers. Both types of bilayers were characterized by ellipsometry, contact angle, and XPS measurements. Symmetric bilayers obtained by the two processes have similar properties.

Local composition, structure, morphology, and phase are interrelated in lipid bilayer membranes. This work describes proofs-of-concept for methods that may be used and combined to modulate, to measure, and to probe the local structure of model membranes through control of membrane curvature in liposomes. These studies are performed using combinations of simultaneous two-color widefield fluorescence imaging, three-dimensional rendering of vesicle domains, and manipulation of the vesicle morphology via micropipet aspiration and the polymerization of cytoskeletal elements inside the membrane. Additionally, we report an electroformation method for preparing unilamellar lipid vesicles that allows hydration of membranes in physiological buffer conditions and formation of multicomponent/multiphase vesicles.

Exposure of self-assembled monolayers to highly charged ions and metastable atomsThe combination of self-, directed, and positional assembly techniques, i.e., ''bottom up'' fabrication, will be essential for patterning and connecting future nanodevices. Systematic exploration of local intermolecular interactions on surfaces will permit their exploitation for the rational design of molecular-scale surface structures. We use the scanning tunneling microscope to probe the local behavior of self-assembled films at the nanometer scale. The ability to control the molecular placement within and by self-assembled monolayers is a means of patterning surfaces. A monolayer with customized features can be produced by manipulating the dynamics of film formation, which are heavily affected by the selectable intermolecular interactions of adsorbates and the structural components naturally occurring within the films. Additionally, the controlled placement and thickness of self-assembled multilayers created from alternating strata of ␣,-mercaptoalkanoic acids and coordinated metal ions can be developed to form precise ''molecular ruler'' resists and to assist in the formation of tailored, lithographically defined metal contacts.

Novel symmetric and asymmetric bilayers, as shown on the right and far right, have been constructed on a gold surface. They are based on octacoordinated metal ions (CeIV, ZrIV, and TiIV) and bifunctional ligands containing a surface immobilization function and a bidentate coordination site. A bilayer with a second bidentate binder may be prepared from preformed complexes or by a stepwise procedure.

Size control in epitaxial Cd(Se,Te) quantum dots (QDs) electrochemically deposited on {HI} textured Au is achieved by mismatch tuning. The formation of QDs as a form of relaxation of heteroepitaxial strain energy is demonstrated. The increased lattice parameter resulting from incorporation of small amounts of Te in the CdSe lattice leads to reduced mismatch-induced strain energy and therefore larger QD size. While the QD interatomic spacings at the interface are shown to be unchanged due to the heteroepitaxy with the Au substrate, the perpendicular QD dspacings (along the c-axis) strongly increase with increasing Te content due to strain relaxation.