For the environmental energy-enhanced interfacial solar vapor generator, through elegant structural designs, there will be a net energy gain from the environment during the solar vapor generation, yielding an evaporation rate exceeding the theoretical value, assuming a 100% solar-to-vapor energy transfer efficiency.

Large-amplitude dynamic force microscopy is used to study alkali halide surfaces. The {001} cleavage faces of NaF, RbBr, LiF, KI and NaCl could be atomically resolved with excellent stability. In all cases the observed lattice periods correspond to the bulk lattice of equally charged ions. The resonance frequency shift and the atomic corrugation amplitude tend to increase after successive tip crashes. This behaviour is explained by analogy with scanning tunnelling microscopy. In addition, the mean atomic corrugation is found to be comparable to the difference between the anion and cation ionic radii.In contact force microscopy, the most established force microscopy technique [1], large contact stresses between tip and sample can cause the surface and/or the tip to be strongly deformed. Atomic scale imaging with sub-unit cell resolution is therefore difficult to achieve [2], especially on strongly reactive surfaces like Si(111)7 × 7 with unsaturated dangling bonds [3]. Giessibl [4] showed that large-amplitude dynamic force microscopy (DFM) is able to overcome these problems. Subsequent experiments on Si (111)7 × 7 [5-8], Si(100)2 × 1 [8], InP(110) [9] and NaCl(001) [10] have established that true atomic resolution can be achieved with this technique on a large variety of materials.In DFM a cantilever is driven with a constant amplitude A ≈ 10-15 nm at the resonance frequency f , and changes of the oscillation properties (frequency shift ∆ f , damping) due to tip-sample interactions are measured.For accurate measurement of ∆ f in UHV, we adapted a fast frequency modulation (FM) scheme 1 similar to that described by Albrecht et al. [11]. Microfabricated single-crystal Si cantilevers (n-doped, 0.01-0.02 Ω cm) of rectangular shape [12], with a bending spring constant k ≈ 30-35 N/m and a resonance frequency f 0 ≈ 164-169 kHz were used.For the present experiments NaF, RbBr, LiF, KI and NaCl single crystals were cleaved in ultrahigh vacuum 1 Our FM detector was built in collaboration with OMICRON Vaku-

The local contact potential difference (CPD) between different linear alkanethiols self-assembled as monolayers on Au substrates was investigated with Kelvin probe force microscopy (KPFM). Our results demonstrate that KPFM simultaneously provides information on the sample topography and its contact potential down to a lateral resolution of 100 nm. The reported CPD measurements allow the distinction between regions in the monolayer comprising thiol molecules with chemically different terminal head groups down to a resolution of 3 meV. Furthermore, the CPD values measured in monolayers having one type of functionality vary with the chain length of the molecules in the film, which is consistent with a dipole-layer model. Variations in the length of the alkyl chain reveal a linear dependence as a function of the (-CH2-) units in the chain, with an increase in the CPD potential of 14.1 ( 3.1 mV per (-CH2-) unit.

Vertical stacking of monolayers via van der Waals assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional (2D) materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so to enhance theoretical tractability and assist in experimental designs. Specifically, the shortrange attractive van der Waals forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these structures. Here we reliably quantify the the strength of van der Waals forces in terms of an effective Hamaker parameter for CVD-grown graphene and show how it scales by a factor of two or three from single to multiple layers on standard supporting surfaces such as copper or silicon oxide. Furthermore, direct measurements on freestanding graphene provide the means to discern the interplay between the van der Waals potential of graphene and its supporting substrate. Our results demonstrated that the underlying substrates could enhance or reduce the van der Waals force of graphene surfaces, and its consequences are explained in terms of a Lifshitz theorybased analytical model.

No abstract

We investigate interaction effects which occur in scanning tunneling microscopy (STM) by performing local force spectroscopy with an oscillating tip while imaging Si(111)7 × 7 terraces in the dynamic lever STM mode (constant time-averaged current). It is found that true atomic resolution is achieved close to the minimum of the resonance frequency vs. distance curve and even closer to the sample. On the other hand true atomic resolution in noncontact AFM (constant frequency shift) is expected several nm away from this minimum, in the range where the frequency shift becomes more negative with decreasing distance.Force microscopy is one of the most successful scanning probe techniques [1]; for an overview see [2]. It has become evident that a force microscope can be operated in particular ways to achieve true atomic resolution, e.g. of point defects and steps [3][4][5]. However, such images have proved extremely difficult to obtain under stable conditions. Recently it has been convincingly demonstrated that this is in fact possible on a doped semiconductor with a doped Si tip in the noncontact dynamic mode using the mean (timeaveraged) tunneling current for distance control and simultaneously measuring the frequency shift of the oscillating cantilever [6,7]. In this "dynamic lever STM" mode, true atomic resolution was achieved on the reconstructed surface of Si(111)7 × 7 even across and along steps. As demonstrated elsewhere [7], only a short distance regime is suitable for stable atomic resolution, just as in conventional STM. At such tip-sample separations (typically a few Å), a mean tunneling current can be detected and incipient chemical interactions yield atomic-scale features in the frequency shift image.Recently, noncontact atomic force microscopy (nc-AFM) using the frequency shift for distance control has also been demonstrated with stable true atomic resolution on a pure insulator [8]. In parallel investigations on n-doped Si(111)7 × 7 [9] we found that the contrast and the underlying mechanisms are different in the dynamic lever STM and nc-AFM imaging modes. One can attempt to determine the influence of interactions causing the frequency shift during STM operation and vice versa, to measure the current variations during nc-AFM operation.An interesting related question is the tip-sample separations at which true atomic resolution can be achieved in the different modes.

In this work, we study the surface energy of monolayer, bilayer and multilayer graphene coatings, produced through exfoliation of natural graphite flakes and chemical vapor deposition.

No abstract