The effects of oxygen-induced phase segregation on the interfacial electronic structure and quantum efficiency of Cs3Sb photocathodes.

High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

Mechanism of Gold-Assisted Exfoliation of Centimeter-Sized Transition-Metal Dichalcogenide Monolayers.

Exfoliation of large-area monolayers is important for fundamental research and technological implementation of transition-metal dichalcogenides. Various techniques have been explored to increase the exfoliation yield, but little is known about the underlying mechanism at the atomic level. Here, we demonstrate gold-assisted mechanical exfoliation of monolayer molybdenum disulfide, up to a centimeter scale. Detailed spectroscopic, microscopic, and first-principles density functional theory analyses reveal that strong van der Waals (vdW) interaction between Au and the topmost MoS2 layer facilitates the exfoliation of monolayers. However, the large-area exfoliation promoted by such strong vdW interaction is only achievable on freshly prepared clean and smooth Au surfaces, while rough surfaces and surfaces exposed to air for more than 15 min result in negligible exfoliation yields. This technique is successfully extended to MoSe2, WS2, WSe2, MoTe2, WTe2, and GaSe. In addition, electrochemical characterization reveals intriguing interactions between monolayer MoS2 and Au. A subnanometer-thick MoS2 monolayer strongly passivates the chemical properties of the underlying Au, and the Au significantly modulates the electronic band structure of the MoS2, turning it from semiconducting to metallic. This could find applications in many areas, including electrochemistry, photovoltaics, and photocatalysis.

High-affinity adsorption leads to molecularly ordered interfaces on TiO2 in air and solution.

Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO2) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO2 selectively adsorbs atmospheric carboxylic acids that are typically present in parts-per-billion concentrations while effectively repelling other adsorbates, such as alcohols, that are present in much higher concentrations. The high affinity of the surface for carboxylic acids is attributed to their bidentate binding. These self-assembled monolayers have the unusual property of being both hydrophobic and highly water-soluble, which may contribute to the self-cleaning properties of TiO2 This finding is relevant to TiO2 photocatalysis, because the self-assembled carboxylate monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis.

Half-flat vs. atomically flat: Alkyl monolayers on morphologically controlled Si(100) and Si(111) have very similar structure, density, and chemical stability.

Chemists have long preferred the Si(111) surface for chemical functionalization, as a simple aqueous etch can be used to produce ideal, atomically flat H/Si(111) surfaces for subsequent reactions. In contrast, industry-standard etches produce rough H/Si(100) surfaces terminated by nanohillocks. The recent discovery of an aqueous etch that produces morphologically controlled H/Si(100) surfaces with a near atomically flat or "half-flat" morphology challenges the assumption that Si(111) is an inherently preferable starting point for chemical functionalization. This study shows that alkyl functionalization of morphologically controlled, "half-flat" Si(100) surfaces by terminal alkenes produces dense, tightly packed monolayers that are essentially identical to those on atomically flat Si(111). The striking similarity between the infrared spectra on these two surfaces - in terms of absolute absorbance, line shape and position, and polarization dependence - strongly suggests that alkyl monolayers on morphologically controlled Si(111) and Si(100) have essentially identical structures. The principle difference between the two surfaces is the amount of residual H at the Si/organic interface, a difference that is dictated by the structure of the Si(100) surface. Alkyl monolayers on morphologically controlled Si(111) and Si(100) surfaces were shown to be equally resistant to harsh oxidants. As a result, there appears to be no chemical reason to prefer one surface over the other, at least for functionalization with terminal alkenes.

Finding needles in haystacks: scanning tunneling microscopy reveals the complex reactivity of Si(100) surfaces.

Many chemical reactions-etching, growth, and catalytic-produce highly faceted surfaces. Examples range from the atomically flat silicon surfaces produced by anisotropic etchants to the wide variety of faceted nanoparticles, including cubes, wires, plates, tetrapods, and more. This faceting is a macroscopic manifestation of highly site-specific surface reactions. In this Account, we show that these site-specific reactions literally write a record of their chemical reactivity in the morphology of the surface-a record that can be quantified with scanning tunneling microscopy. Paradoxically, the sites targeted by these highly site-specific reactions are extremely rare. This paradox can be understood from a simple kinetic argument. An etchant that produces atomically flat surfaces must rapidly etch every surface site except the terrace atoms on the perfectly flat surface. As a result, the etch morphology is dominated by the least reactive species (here, the terrace sites), not the most reactive species. In contrast, the most interesting chemical species-the site where the reaction occurs most rapidly and most selectively-is the hardest one to find. This highly reactive site, the key to the reaction, is the needle in the haystack, often occurring in densities far below 1% of a monolayer and thus invisible to surface spectroscopies. This kinetic argument is quite general and applies to a wide variety of reactions, not just etching reactions. Understanding these highly site-specific reactions requires a combination of experimental and computational techniques with both exquisite defect sensitivity and high chemical sensitivity. In this Account, we present examples of highly site-specific chemistry on the technologically important face of silicon, Si(100). In one example, we show that the high reactivity of one particular surface site, a silicon dihydride bound to a silicon monohydride, or an "α-dihydride", provides a fundamental explanation for anisotropic silicon etching, a technology widely used in micromachining to selectively produce flat Si{111} surfaces. Fast-etching surfaces, such as Si(100) and Si(110), have geometries that support autocatalytic etching of α-dihydrides. In contrast, α-dihydrides exist only at kink sites on Si(111) surfaces. As a result, the etch rate of surfaces vicinal to Si(111) scales with the step density, approaching zero on the atomically flat surface. In a second example, we explain the chemistry that underlies pyramidal texturing of silicon wafers, a technique that is sometimes used to decrease the reflectivity of silicon solar cells. We show that a subtle change in chemical reactivity transforms a near-perfect Si(100) etchant into one that spontaneously produces nanoscale pyramids. The pyramids are not static features; they are self-propagating structures that evolve in size and location as the etching proceeds. The key to this texturing is the production of a very rare defect at the apex of each pyramid, a site that also etches autocatalytically. These experiments show that simple chemical reactions can enable an exquisite degree of atomic-scale control if only we can learn to harness them.

Kinetic Monte Carlo simulations of anisotropic Si(100) etching: Modeling the chemical origins of characteristic etch morphologies.

An atomistic, chemically realistic, kinetic Monte Carlo simulator of anisotropic Si(100) etching was developed. Surface silicon atoms were classified on the basis of their local structure, and all atoms of each class were etched with the same rate. A wide variety of morphologies, including rough, striped, and hillocked, was observed. General reactivity trends were correlated with specific morphological features. The production of long rows of unstrained dihydride species, recently observed in NH(4)F (aq) etching of Si(100), could only be explained by the rapid etching of dihydrides that are adjacent to (strained) monohydrides-so-called "alpha-dihydrides." Some etch kinetics promoted the formation of {111}-microfaceted pyramidal hillocks, similar in structure to those observed experimentally during Si(100) etching. Pyramid formation was intrinsic to the etch kinetics. In contrast with previously postulated mechanisms of pyramid formation, no masking agent (e.g., impurity, gas bubble) was required. Pyramid formation was explained in terms of the slow etch rate of the {111} sides, {110} edges, and the dihydride species that terminated the apex of the pyramid. As a result, slow etching of Si(111) surfaces was a necessary, but insufficient, criterion for microfacet formation on Si(100) surfaces.

Extracting maximum information from polarized surface vibrational spectra: application to etched, H-terminated Si(110) surfaces.

A general method to maximize the information extracted from polarized surface absorption spectra is developed and applied to the study of etched Si(110) surfaces. In essence, this technique transforms spectra from the experimental reference frame, which is defined by the direction of the surface electric field during irradiation by s- and p-polarized light, into a more appropriate Cartesian reference frame defined by the surface normal and the plane of incidence. If the Cartesian reference frame is aligned with high symmetry directions of the system, significant spectral simplification can result. This analysis relies on the well-known boundary conditions on interfacial electric fields and is independent of any adsorbate screening or the effective dielectric constant of the adsorbate layer. The validity of this analysis is demonstrated on the spectra of NH4F-etched, H-terminated Si(110). The transition dipole moments of the symmetric and antisymmetric Si[Single Bond]H stretch modes associated with flat terraces are polarized along the [110] and [001] directions, respectively. Two additional modes with transition dipoles polarized along the [001] and [110] directions are assigned to defect species associated with microfaceting and other surface roughness. Data taken in two different experimental geometries are shown to be in excellent quantitative agreement, confirming the validity of the technique. Additionally, the measured adsorbate layer dielectric constant is in good agreement with previously reported values for hydrogen-terminated silicon surfaces.

Understanding the effects of surface chemistry on Q: mechanical energy dissipation in alkyl-terminated (C1-C18) micromechanical silicon resonators.

The rate of mechanical energy dissipation in 300-nm-thick, megahertz-range micromechanical silicon resonators is sensitive to single monolayer changes in surface chemistry. Resonators terminated with a single monolayer of methyl groups have significantly higher quality factors (Q's), and thus lower mechanical energy dissipation, than comparable resonators terminated with either long-chain alkyl monolayers (C2H2n+1, n = 2-18) or monolayers of hydrogen atoms. This effect cannot be attributed to mechanical energy dissipation within the alkyl monolayer, as a 9-fold increase in alkyl chain length does not lead to an observable increase in dissipation. Similarly, this effect is not correlated with the chemical structure of the silicon-monolayer interface (e.g., the density of Si-H vs Si-C bonds.) Instead, the chemical trends in resonator quality and stability are consistent with a dissipation mechanism involving the coupling of long-range strain fields to localized, electronically active defects in the monolayer coatings.

Production of highly homogeneous Si(100) surfaces by H2O etching: surface morphology and the role of strain.

The etching of Si(100) surfaces in ultrapure water was studied with a combination of infrared spectroscopy (FTIR) and scanning tunneling microscopy (STM). While the FTIR results show that the initially rough H/Si(100) surface becomes highly homogeneous during etching, a phenomenon generally associated with surface smoothing, STM images reveal that the homogeneity is associated with the formation of well-defined etch hillocks. After many hours of etching, the resulting H-terminated surface is composed of stripes of atomically flat Si(100) terminated by etch hillocks bounded by {111}- and {110}-oriented microfacets. Polarization analysis of the Si-H stretching modes provides strong evidence for uniform dihydride-termination of the flat regions, with the narrow (approximately 25 A) width of these stripes allowing for relaxation of steric strain between neighboring dihydrides. The unusual hill-and-valley etch morphology is attributed to the effects of steric strain on the reactivity of sites on the etched surface.

Etchant anisotropy controls the step bunching instability in KOH etching of silicon.

STM investigations of vicinal Si(111) surfaces etched in KOH solutions under controlled flow conditions show that step bunching instability is due to inhomogeneities that develop in the etchant as the result of highly step-site-specific etching reactions. Other previously postulated mechanisms for step bunching, including anisotropic surface diffusion, surface strain, and impurity deposition, are conclusively ruled out. The inhomogeneities locally accelerate etching near surface steps. Kinetic Monte Carlo simulations of this process qualitatively reproduce the observed morphologies.

In search of perfection: understanding the highly defect-selective chemistry of anisotropic etching.

Anisotropic etchants selectively reveal a specific crystallographic plane. Although prized industrially, these etchants are poorly understood because they target specific defect sites on a surface. New methods, which rely on a combination of scanning tunneling microscopy, kinetic Monte Carlo simulations, and infrared spectroscopy, have been developed to quantify these reactions. By correlating the measured reaction rates with the structure of the defects, information about reaction mechanisms can be obtained. These techniques have also been extended to allow for the quantification of impurity reactions such as the reaction of dissolved O2, and of nonetching additives, such as alcohols. A complementary macroscopic technique, which utilizes microfabricated arrays of miscut surfaces to measure orientation-dependent kinetics, is also described.

Differential binding of host complement inhibitor factor H by Borrelia burgdorferi Erp surface proteins: a possible mechanism underlying the expansive host range of Lyme disease spirochetes.

The Lyme disease spirochete, Borrelia burgdorferi, is capable of infecting a wide variety of vertebrates. This broad host range implies that B. burgdorferi possesses the ability to contravene the immune defenses of many potential hosts. B. burgdorferi produces multiple different Erp proteins on its outer membrane during mammalian infection. It was reported previously that one Erp protein can bind human factor H (J. Hellwage, T. Meri, T. Heikkilä, A. Alitalo, J. Panelius, P. Lahdenne, I. J. T. Seppälä, and S. Meri, J. Biol. Chem. 276:8427-8435, 2001). In this paper we report that the ability to bind the complement inhibitor factor H is a general characteristic of Erp proteins. Furthermore, each Erp protein exhibits different relative affinities for the complement inhibitors of various potential animal hosts. The data suggest that the presence of multiple Erp proteins on the surface can allow a single B. burgdorferi bacterium to resist complement-mediated killing in any of the wide range of potential hosts that it might infect. Thus, Erp proteins likely contribute to the persistence of B. burgdorferi in nature and to the ability of this bacterium to cause Lyme disease in humans and other animals.