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Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one distinct region of the moiré supercell. Scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements show that a highly periodic hydrogen functionalized graphene sheet can thus be prepared by controlling the sample temperature (T) during hydrogen functionalization. At deposition temperatures of T = 645 K and above, hydrogen adsorbs exclusively on the HCP regions of the graphene/Ir(111) moiré structure. This finding is rationalized in terms of a slight preference for hydrogen clusters in the HCP regions over the FCC regions, as found by density functional theory calculations. Angle-resolved photoemission spectroscopy measurements demonstrate that the preferential functionalization of just one region of the moiré supercell results in a band gap opening with very limited associated band broadening. Thus, hydrogenation at elevated sample temperatures provides a pathway to efficient band gap engineering in graphene via the selective functionalization of specific regions of the moiré structure.

We report here the discovery of a new form of spontaneously polarized material. Examples of this material, in the form of films, demonstrate the property that they spontaneously harbour electric fields which may exceed 10(8) Vm(-1), achieving potentials of tens of volts on the film surface. The molecules presently identified form a diverse group, thus far of six species, with gas phase dipoles lying between 0.08 D and 0.5 D: propane (0.08 D), isopentane (0.13 D), nitrous oxide (0.167 D), isoprene (0.25 D), toluene (0.385 D) and CF(3)Cl (Freon-13; 0.5 D). Here we concentrate on an understanding of the nature of the interactions which give rise to the spontaneously polarized state, using the measured temperature dependence of the electric field in N(2)O as a diagnostic. We show that the polarized state can arise through a mechanism of non-linear dipole alignment in a single domain in which dipole alignment generates the electric field within the film and the field generates dipole alignment. Non-local interactions take place over the dimension of the thickness of the film and permeate the entire medium through the agency of the electric field. This new type of material may have wide ranging applications in devices and in nanotechnology.

A recent publication described a new group of spontaneously polarized materials in which electric fields in excess of 10(8) V m(-1) may be present. This phenomenon arises through dipole alignment in solid films formed by straightforward deposition from vapour and characterises a novel class of materials. Here we present further results for the properties of these materials, focusing on films of cis-methyl formate. These films are shown to display some notable new chemical physics. We find the novel result that the degree of dipole alignment and the corresponding electric field in films of cis-methyl formate can have a counter-intuitive temperature dependence, increasing six-fold between 80 K and 89 K, in sharp contrast to the pronounced and expected fall with deposition temperature seen both here between 50 K and 75 K and in numerous other species. A theoretical model demonstrates that the switch of gradient with rising temperature should be a general phenomenon and is associated with crossing of a singularity in the gradient occurring at a set of critical values of temperature and alignment.

Reflection-absorption infrared spectroscopy (RAIRS) is shown to provide a means of observing the spontelectric phase of matter, the defining characteristic of which is the occurrence of a spontaneous and powerful static electric field within a film of material. The presence of such a field is demonstrated here through the study of longitudinal-transverse optical splitting in RAIR spectra in films of carbon monoxide, based upon the deposition temperature dependence of this splitting. Analysis of spectral data, in terms of the vibrational Stark effect, allows the measurement of the polarization of spontelectric films, showing for example that solid carbon monoxide at 20 K may maintain a spontelectric field of 3.78 × 10(7) V m(-1), representing a polarization of 3.34 × 10(-4) cm(-2). We comment on the astrophysical implications of polarized carbon monoxide ices, on the surface of cosmic grains in star-forming regions.

Reflection-absorption infrared spectroscopy (RAIRS) of nitrous oxide (N2O) thin films is shown to provide an independent means of observing the spontelectric state, the first new structural phase of matter, with unique electrical properties, to have emerged in decades. The presence of a spontaneous and powerful static electric field within the film, the defining characteristic of spontelectric solids, is demonstrated through observations of longitudinal-transverse optical (LO-TO) splitting in RAIR spectra, using an analysis based on the vibrational Stark effect. In particular the dependence of the LO-TO splitting on the film deposition temperature may be wholly attributed to the known temperature dependence of the spontelectric field.

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The recent discovery of a new class of solids displaying bulk spontaneous electric fields as high as 10(8) V m(-1), so-called 'spontelectrics', poses fundamental and unresolved problems in solid state physics. The purpose of the present work is to delve more deeply into the nature of the interactions which give rise to the spontelectric effect in films of nitrous oxide (N2O), by observing the variation of the spontaneous field as the N2O molecules are physically removed from one another by dilution in Xe. Data, obtained using the ASTRID storage ring, are presented for films diluted by factors ξ = Xe/N2O of 0.9 to 67, at deposition temperatures of 38 K, 44 K and 48 K, where films are laid down by deposition from a gas mixture. Results show that the spontelectric field decreases as ξ increases and that at ξ = 67 for 44 K deposition, the spontelectric effect is absent. Reflection-absorption infrared spectroscopy (RAIRS) data are also reported, providing insight into the structure of Xe/N2O films and specifically showing that N2O remains dispersed in the Xe/N2O films prepared here. A simplified theoretical model is developed which illustrates that electric fields can be understood in terms of dilution-dependent dipole orientation. This model is used to reproduce experimental data up to an average molecular separation, s, of ≥1.25 nm apart, ∼4 times that associated with pure solid N2O. The disappearance of the spontelectric effect at larger average distances of separation, between s = 1.25 nm and s = 1.75 nm, is a phenomenon which cannot be described by any existing model but which shows that dipole-dipole interactions are an essential ingredient for the creation of the spontelectric state.