Chlorine Monolayer Research Articles

The role of chlorine promoters in mediating particle size effects in silver-catalyzed ethylene epoxidation

Measured chlorine coverages over Ag/α-Al2O3 catalysts with varying Ag weight loadings (1.3–35 wt%) and different Ag particle size distributions when parsed in terms of Cl coverages over particles of varying size constituted in the distribution reveal that Ag particles of size below ∼30 nm are covered in more than 1 monolayer (ML) of chlorine in presence of 3.5 ppm C2H5Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al2O3 samples using a single exponential decay function. Fully chlorided small Ag particles exhibit low rates of epoxidation and only large Ag particles which exhibit sub-monolayer Cl coverages catalyze ethylene epoxidation with high ethylene oxide (EO) rates and selectivity. The Ag particle size dependence of Cl coverages plausibly explains why ≥ 100 nm Ag particles are typical in promoted Ag/α-Al2O3 ethylene epoxidation catalysts.

PBr3 adsorption on a chlorinated Si(100) surface with mono- and bivacancies.

For the most precise incorporation of single impurities in silicon, which is utilized to create quantum devices, a monolayer of adatoms on the Si(100) surface and a dopant-containing molecule are used. Here, we studied the interaction of phosphorus tribromide with a chlorine monolayer with mono- and bivacancies using a scanning tunneling microscope (STM) at 77K. The combination of different halogens in the molecule and the adsorbate layer enabled unambiguous identification of the structures after PBr3 dissociation on Si(100)-Cl. A Cl monolayer was exposed to PBr3 in the STM chamber, which allows us to compare the same surface areas before and after PBr3 adsorption. As a result of this comparison, we detected small changes in the chlorine layer and unraveled the molecular fragments filling mono- and bivacancies. Using density functional theory, we found that the phosphorus atom occupies a bridge position after dissociation of the PBr3 molecule, which primarily bonds with silicon in Cl bivacancies. These findings provide insight into the interaction of a dopant-containing molecule with an adsorbate monolayer on Si(100) and can be applied to improve the process of single impurity incorporation into silicon.

Reactivity of the Si(100)-2 × 1-Cl surface with respect to PH3, PCl3, and BCl3: comparison with PH3 on Si(100)-2 × 1-H

Despite the interest in a chlorine monolayer on Si(100) as an alternative to hydrogen resist for atomic-precision doping, little is known about its interaction with dopant-containing molecules. We used the density functional theory to evaluate whether a chlorine monolayer on Si(100) is suitable as a resist for PH3, PCl3, and BCl3 molecules. We calculated reaction pathways for PH3, PCl3, and BCl3 adsorption on a bare and Cl-terminated Si(100)-2 × 1 surface, as well as for PH3 adsorption on H-terminated Si(100)-2 × 1, which is widely used in current technologies for atomically precise doping of Si(100) with phosphorus. It was found that the Si(100)-2 × 1-Cl surface has a higher reactivity toward phosphine than Si(100)-2 × 1-H, and, therefore, unpatterned areas are less protected from undesirable incorporation of PH3 fragments. On the contrary, the resistance of the Si(100)-2 × 1-Cl surface against the chlorine-containing molecules turned out to be very high. Several factors influencing reactivity are discussed. The results reveal that phosphorus and boron trichlorides are well-suited for doping a patterned Cl-resist by donors and acceptors, respectively.

Designer flat bands in quasi-one-dimensional atomic lattices

Certain lattices with specific geometries have one or more spectral bands that are strictly flat, i.e. the electron energy is independent of the momentum. This can occur robustly irrespective of the specific couplings between the lattices sites due to the lattice symmetry, or it can result from fine-tuned couplings between the lattice sites. While the theoretical picture behind flat electronic bands is well-developed, experimental realization of these lattices has proven challenging. Utilizing scanning tunnelling microscopy (STM) and spectroscopy (STS), we manipulate individual vacancies in a chlorine monolayer on Cu(100) to construct various atomically precise 1D lattices with engineered flat bands. We realize experimentally both gapped and gapless flat band systems with single or multiple flat bands. We also demonstrate tuneability of the energy of the flat bands and how they can be switched "on" and "off" by breaking and restoring the symmetry of the lattice geometry. The experimental findings are corroborated by tight-binding calculations. Our results constitute the first experimental realizations of engineered flat bands in a 1D solid-state system and pave the way towards the construction of e.g. topological flat band systems and experimental tests of flat-band-assisted superconductivity in a fully controlled system.

Chlorine insertion and manipulation on the Si(100)- 2×1 -Cl surface in the regime of local supersaturation

We insert and manipulate a single chlorine atom in chlorine monolayer on a Si(100)-2x1 surface using a scanning tunneling microscope. Two objects were created - a Cl atom in a groove between two dimer rows, and bridge-bonded Cl on a silicon dimer. Changing the voltage polarity leads to conversion of the objects into each other. Anisotropic movement of the objects at 77 K is mediated by two different diffusion mechanisms: hopping and crowdion-like motion. Insertion of a Cl atom in a groove between two dimer rows leads to the formation of a dangling bond on a third-layer Si atom. At positive sample voltage bias, the first object is positively charged, while the second object can be neutral or negatively charged depending on silicon sample doping.

Kinetic Modeling of Ethylene Epoxidation Kinetics as a Function of Chlorine Coverage over a Highly Promoted Silver Catalyst

Kinetic modeling of previously reported data demonstrate that chlorine exhibits an electrostatic influence, rather than solely ensemble site blocking, to improve the selectivity of ethylene epoxidation over promoted silver catalysts. Kinetic models that neglect the impact of surface chlorine content on either oxygen dissociation or ethylene adsorption enthalpies are unable to describe observed changes in the apparent reaction orders of ethylene consumption. Models that employ solely ensemble effects to adjust product selectivity with chlorine content falsely predict a simultaneous maximum in ethylene oxide (EO) and carbon dioxide synthesis rates with respect to chlorine coverage at 10–15% of a monolayer of chlorine. Incorporating electronic effects of chlorine on the transition states of the selectivity-determining steps into the model, however, properly describes the measured rate trends with surface chlorine content. Specifically, the model reflects the maxima observed in ethylene oxide synthesis rate with the concomitant monotonic decrease in combustion rates with increasing chloride content. Slight improvements to model predictions by combining ensemble and electronic effects on EO selectivity with increasing Cl coverage suggest that both effects may be relevant in combination but that ensemble effects alone cannot describe the observed behavior.

Ab Initio Study of the Early Stage of Si Epitaxy on the Chlorinated Si(100) Surface

The homoepitaxial growth of Si on Si(100) covered by a resist mask is a necessary technological step for the fabrication of donor-based quantum devices with scanning tunneling microscope lithography. In the present work, the chlorine monolayer is selected as the resist. Using density functional theory, we investigated the adsorption of a single silicon atom on Si(100)-2$\times$1-Cl as the starting process of Si epitaxy. The incorporation of a silicon atom under a Cl monolayer proved to be the most energetically favorable process. Our results show that chlorine segregates on the surface during Si deposition and does not incorporate into homoepitaxial layers. In addition, we found that SiCl$_2^{\ast}$, SiCl$_3^{\ast}$, and SiCl$_4^{\ast}$ clusters can be formed above a Si(100)-2$\times$1-Cl surface while Si is adsorbed. SiCl$_2^{\ast}$ clusters are bound weakly to the substrate, and their desorption leaves the silicon surface free of chlorine. To check whether the Si epitaxy is possible on the chlorine resist, we compare our results with the well-studied case of a hydrogen resist. We find the two processes to be similar; moreover, epitaxy on chlorine resist appears to have an advantage.

First-Principle Study of Phosphine Adsorption on Si(001)-2 × 1–Cl

This paper presents a DFT study for phosphine adsorption on a Si(001)-2$\times$1 surface covered by a chlorine monolayer, including adsorption on local defects, i.e. mono- and bivacancies in the adsorbate layer (Cl, Cl$_2$), and combined vacancies with removed silicon atoms (SiCl, SiCl$_2$). Activation barriers were found for the adsorbing PH$_3$ to dissociate into PH$_2$+H and PH+H$_2$ fragments; it was also established that phosphine dissociation on combined vacancies is possible at room temperature. If there is a silicon vacancy on the surface, phosphorus settles in the Si(001) lattice as PH (if the vacancy is SiCl) or as PH$_2$ (if the vacancy is SiCl$_2$). This paper suggests a method to plant a separate phosphorus atom into the silicon surface layer with atomic precision, using phosphine adsorption on defects specially created on a Si(001)-2$\times$1-Cl surface with an STM tip.

High Curie-temperature intrinsic ferromagnetism and hole doping-induced half-metallicity in two-dimensional scandium chlorine monolayers.

Two-dimensional (2D) ferromagnetic materials provide new platforms for spintronic applications, but most of the reported 2D magnetic orderings can only be maintained at low temperature. The search for high Curie temperature intrinsic ferromagnetism is still a current hotspot. Herein through comprehensive first-principles calculations and Monte Carlo simulation, we predict a promising 2D scandium chlorine (ScCl) monolayer with intrinsic ferromagnetism and a Curie temperature of 185 K, which is much higher than that of the reported CrI3 monolayer (45 K) and the boiling point of liquid nitrogen (77 K). Moreover, a small amount of hole doping can induce a transition from a ferromagnetic metal to a half-metal. Furthermore, the ScCl monolayer possesses excellent thermal and dynamical stabilities as well as feasibility of experimental exfoliation from its layered bulk. These intriguing electronic and magnetic properties make the ScCl monolayer a promising candidate for spintronic applications.

Emergence of quasiparticle Bloch states in artificial crystals crafted atom-by-atom

The interaction of electrons with a periodic potential of atoms in crystalline solids gives rise to band structure. The band structure of existing materials can be measured by photoemission spectroscopy and accurately understood in terms of the tight-binding model, however not many experimental approaches exist that allow to tailor artificial crystal lattices using a bottom-up approach. The ability to engineer and study atomically crafted designer materials by scanning tunnelling microscopy and spectroscopy (STM/STS) helps to understand the emergence of material properties. Here, we use atom manipulation of individual vacancies in a chlorine monolayer on Cu(100) to construct one- and two-dimensional structures of various densities and sizes. Local STS measurements reveal the emergence of quasiparticle bands, evidenced by standing Bloch waves, with tuneable dispersion. The experimental data are understood in terms of a tight-binding model combined with an additional broadening term that allows an estimation of the coupling to the underlying substrate.

Topological states in engineered atomic lattices

Topological materials exhibit protected edge modes that have been proposed for applications in for example spintronics and quantum computation. While a number of such systems exist, it would be desirable to be able to test theoretical proposals in an artificial system that allows precise control over the key parameters of the model. The essential physics of several topological systems can be captured by tight-binding models, which can also be implemented in artificial lattices. Here, we show that this method can be realized in a vacancy lattice in a chlorine monolayer on a Cu(100) surface. We use low-temperature scanning tunneling microscopy (STM) to fabricate such lattices with atomic precision and probe the resulting local density of states (LDOS) with scanning tunneling spectroscopy (STS). We create analogues of two tight-binding models of fundamental importance: The polyacetylene (dimer) chain with topological domain wall states, and the Lieb lattice, featuring a lattice pseudospin 1 system with a flat electron band. These results provide the first steps in realizing designer quantum materials with tailored properties.

Analysis of temporal evolution of quantum dot surface chemistry by surface-enhanced Raman scattering.

Temporal evolution of surface chemistry during oxidation of silicon quantum dot (Si-QD) surfaces were probed using surface-enhanced Raman scattering (SERS). A monolayer of hydrogen and chlorine terminated plasma-synthesized Si-QDs were spin-coated on silver oxide thin films. A clearly enhanced signal of surface modes, including Si-Clx and Si-Hx modes were observed from as-synthesized Si-QDs as a result of the plasmonic enhancement of the Raman signal at Si-QD/silver oxide interface. Upon oxidation, a gradual decrease of Si-Clx and Si-Hx modes, and an emergence of Si-Ox and Si-O-Hx modes have been observed. In addition, first, second and third transverse optical modes of Si-QDs were also observed in the SERS spectra, revealing information on the crystalline morphology of Si-QDs. An absence of any of the abovementioned spectral features, but only the first transverse optical mode of Si-QDs from thick Si-QD films validated that the spectral features observed from Si-QDs on silver oxide thin films are originated from the SERS effect. These results indicate that real-time SERS is a powerful diagnostic tool and a novel approach to probe the dynamic surface/interface chemistry of quantum dots, especially when they involve in oxidative, catalytic, and electrochemical surface/interface reactions.

First-Principle Study of Adsorption and Desorption of Chlorine on Cu(111) Surface: Does Chlorine or Copper Chloride Desorb?

First-principle density-functional calculations have been applied to study the interaction of molecular chlorine with the (111) plane of copper. Using transition-state search method, we considered the elementary processes (Cl$_2$ dissociation, adsorption, diffusion, association and desorption) on the chlorinated Cu(111) surface. A systematic study of possible desorption pathways has been carried out for different species (Cl, Cl$_2$, CuCl, CuCl$_2$, and Cu) at various chlorine coverage. As a result, we concluded that chlorine monolayer irrespective of the coverage desorbs in the form of CuCl molecules from step edges.

Silicate-free growth of high-quality ultrathin cerium oxide films on Si(111)

Ultrathin Ce${}_{2}$O${}_{3}$ layers have been grown on Si(111) by reactive metal deposition in an oxygen background and characterized by x-ray standing waves, x-ray diffraction, x-ray photoelectron spectroscopy, and low-energy electron diffraction to elucidate and quantify both atomic structure and chemical composition. It is demonstrated that highly ordered, mostly $B$-oriented, epitaxial ceria films can be achieved by preadsorption of a monolayer of atomic chlorine, effectively passivating the substrate and thereby suppressing cerium silicate and silicon oxide formation at the interface.

Commensurate-incommensurate phase transition in chlorine monolayer chemisorbed on Ag(111): Direct observation of crowdion condensation into a domain-wall fluid

The commensurate-incommensurate phase transition in chlorine monolayer on Ag(111) has been studied with low-temperature scanning tunneling microscopy and low-energy electron diffraction. We report the observation of self-interstitial defects: two-dimensional (2D) crowdions formed at the initial stage of the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R30${}^{\ensuremath{\circ}}$ lattice compression. After critical coverage of 0.34 monolayer (ML) the crowdions condense into a domain-wall fluid. As coverage further increases, the domain-wall crystal solidifies with average wall separation of 23 \AA{}.

Resistless patterning of a chlorine monolayer on a Si(0 0 1) surface with an electron beam

We achieved electron beam (e-beam) patterning without a photoresist on a Cl-terminated Si(0 0 1) surface. Synchrotron radiation photoemission spectroscopy and scanning photoelectron microscopy were employed to investigate the surface chemical state and pattern formation. The Cl–Si bonds were easily broken by the irradiation with an e-beam of 1 keV, leading to a pattern formation through the adsorption of residual molecules of water and hydrocarbon at the exposed Si dangling bond sites. In addition, we demonstrated the selective adsorption of desired molecules on the surface by e-beam irradiation in environments consisting of different gases, such as oxygen, ammonia, and 1-butanethiol.

Mass spectrometry of chlorine monolayers on solid substrates

A new method is proposed for determining chlorine traces in monolayer films adsorbed on solid substrates. The method is based on secondary-ion mass spectrometry (SIMS) with the formation of positive chlorine-containing molecular ions. In contrast to the traditional method making use of inert gases, the secondary ions are generated by bombarding the chlorine-containing substrate with a beam of methane molecules. This leads to the formation of CCl 2 + ions with molecular masses of 82, 84, and 85 amu. These peaks are reliably observed in the form of a doublet not masked by the peaks of ions of the substrate material and the other background reaction products.

The role of O and Cl adsorbates on the secondary emission properties of tungsten

Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na + incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na + impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum.

Direct STM observation of surface modification and growth of AgCl islands on Ag(111) upon chlorination at room temperature

The results of STM, AES and TDS study of a Ag(111) surface treated by molecular chlorine in UHV with doses up to 100 000 L are presented. We obtained direct evidence of AgCl island formation on the surface upon chlorine adsorption at room temperature. The specific feature of the modified silver surface is AgCl islands surrounded by ditches with 50–150 Å depth and untreated atomic plateaux. The atomic structure and chemical state of the surface area with no AgCl islands corresponds to saturated chlorine monolayer with a Ag(111)-(17×17)-Cl structure. A strong influence of AES primary electron beam on the morphology of the silver chloride film was observed by STM. A small (0.1 C cm −2) electron beam dose caused a tremendous modification of the AgCl island film forming ‘giant’ AgCl islands surrounded by a smooth surface covered by chemisorbed chlorine. This surface modification could be an origin of discrepancy in the interpretation of previous electron spectroscopy results.

The adsorption of carbon tetrachloride on Ni(110): an EELS, AES and LEED study

Carbon tetrachloride dissociatively adsorbs on Ni(110) at 293 K forming a mixed monolayer of chlorine and carbon atoms. We assign a nickel–chlorine stretching band at 340 cm −1 to chlorine atoms in B 5 surface sites coordinated to four surface nickel atoms and to one nickel atom in the second layer. These sites are preferentially populated but at high coverages ( θ≥0.6) an almost square chlorine atom mesh forms which we suggest is stabilized by regularly placed carbon atoms. In this square mesh, which is rotated by 43° with respect to the metal mesh, chlorine atoms cannot exclusively occupy B 5 sites and the EEL spectrum shows a shoulder at 240 cm −1 on the side of the 340 cm −1 peak. An analysis of the intensity of the Ni–Cl stretching EELS peak as a function of chlorine coverage shows unusual behaviour which we suggest is associated with the interaction of carbon atoms with chlorine. We estimate the effective charge of the Ni–Cl stretch to be 0.4 e for the saturated monolayer, and significantly higher at very low coverage. Carbon is removed by dissolution on warming to 758 K resulting in a c(2×2) chlorine layer which is typical of pure adsorbed chlorine layers. Chlorine desorbed, probably as a nickel chloride, on heating above 990 K and a clean surface resulted at 1150 K. Adsorption at 108 K was also completely dissociative in the first layer but adsorption of molecular CCl 4 continued into a second and subsequent layers.