Nucleation Of Monolayers Research Articles

Spectroscopic visualization of intermediate phases during CVD synthesis of MoS2

Chemical vapor deposition (CVD) is the broadly used technique in the synthesis of 2D materials for various electronic and optoelectronic devices. In this study, we aim to understand the precise phase transition of molybdenum trioxide (MoO3), the most commonly used transition metal precursor, under the influence of sulfur in a CVD process that ultimately leads to the growth of mono-to few-layer molybdenum disulfide (MoS2) with varying morphologies. For this, we employed a space-confined growth method and carried out a methodical spectroscopic analysis to understand the step-by-step reduction of MoO3 under reaction with sulfur vapors and its subsequent transformation to MoS2 on a SiO2 substrate. The sulfur-promoted reduction of MoO3 forming molybdenum dioxide (MoO2), followed by molybdenum oxysulfide (MoOS2) to MoOS2/MoS2 hybrid crystals, and finally monolayer (few layer) triangle (hexagonal) MoS2 crystals, is discussed. The results show that the nucleation presumably initiated by oxy-chalcogenide micro-particles that serve as heterogeneous monolayer nucleation sites. We validate our findings with optical and scanning electron microscope images, micro-Raman, photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) studies. Our findings can contribute to a better understanding of the growth processes of sequential sulfurization reactions.

Controlled nucleation of monolayer MoSe2 islands on Si (111) by MBE

Van der Waals bonding relaxes the constraints of lattice matching, making two-dimensional (2D) transition metal dichalcogenides attractive in the field of epitaxy. Recently, molecular beam epitaxy (MBE) of MoSe2 was demonstrated on a variety of substrates. Here, the authors use MBE to investigate the early stages of 2D nucleation of MoSe2 grown on Si in pursuit of controlled monolayer island size. The 2D nucleation rate varies by a factor of >2 over a narrow substrate temperature range of 550–560 °C. Above 560 °C, the desorption rate of Se from the surface exceeds the nucleation rate leading to fully suppressed 2D monolayer nucleation. X-ray diffraction confirms (001) oriented MoSe2 on Si (111). Raman spectra are consistent with 1–3 monolayer-thick MoSe2, in agreement with atomic force microscopy measurements of the monolayer height of 2D islands.

(Invited) Phthalocyanine Monolayer Nucleation of Gate Oxide ALD on Single Layer Graphene and TMD Surfaces

Several proposed beyond-CMOS devices based on two-dimensional (2D) heterostructures require the deposition of thin dielectrics between 2D semiconductor layers as an interlayer, or on the 2D semiconductor as a gate. However the direct deposition of dielectrics on 2D materials is challenging due to their inert surface chemistry. To deposit subnanometer EOT dielectrics on 2D semiconductors, a flat lying titanyl phthalocyanine (TiOPc) monolayer was employed to functionalize the 2D materials via non-bonding interaction for atomic layer deposition (ALD), and the initial stage of growth was probed using in-situ STM. ALD pulses of trimethyl aluminum and H2O resulted in the uniform deposition of AlOx on the TiOPc/HOPG via dissociative chemisorption onto TiOPc reactive sites. The uniformity of the dielectric is consistent with DFT calculations showing multiple reaction sites on each TiOPc molecule for reaction with TMA. In order to deposit dielectrics with low leakage and low equivalent oxide thickness (EOT) on WSe2, a flat-lying titanyl phthalocyanine (TiOPc) monolayer (ML), deposited via molecular beam epitaxy (MBE), was integrated to create a seed layer for atomic layer deposition (ALD) on WSe2 surfaces. TiOPc molecules formed a flat lying monolayer on WSe2 ML, with four fold crystal structure in scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O resulted in the uniform deposition of Al2O3 on TiOPc/WSe2, confirmed by AFM and cross sectional TEM. After deposition of 50 cycles of the Al2O3 onto the TiOPc/multilayer WSe2, top gated FETs were fabricated. The top gated leakage current was measured 0.046 pA/µm2 at 1 V gate bias with 2.4 nm EOT, which is lower leakage current than previous results. A subthreshold swing of 80 mV/dec was measured in n-branch of dual gated device. Figure 1

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

Monolayer Catalyst Design Using Slrr of UPD Monolayers – Fundamental Aspects and Interplay of UPD, Reaction Kinetics, and Nucleation Kinetics

The deposition protocol based on surface limited Redox replacement reaction (SLRR) of UPD monolayers [[i]] has gained a lot of attention in recent years. It has been extensively used for preparation of highly active catalysts [[ii],[iii],[iv],[v],[vi],[vii]], and for growth of homo/heteroepitaxial 2D films with arbitrary thickness [[viii],[ix]]. The concept behind this deposition protocol has potential for a wide range of applications, yet very little is known about the governing phenomena determining morphology of deposited catalyst monolayers.In this talk we will review fundamental relations controlling the catalyst monolayer nucleation as a prelude for its morphology design. The analytical model describing the nucleation density as a function on reaction stoichiometry, rate constant, reaction order, and fundamental parameters describing surface transport will be presented and discussed with reflection to experimental data. We will show the practical relevance of SLRR reaction kinetics on deposition flux and nucleation kinetics of metal/catalyst monolayers. The different approaches for manipulation of the reaction kinetics and thermodynamics will be discussed within the frame of experimental design/protocols to control the morphology of deposited catalyst monolayers. The emphasis will be made on some convenient and yet allowed approximation relevant to the real systems.In the second part of the talk the spectroscopic data will be presented emphasizing that catalyst morphology i.e. nanocluster size is an important design parameter directly affecting the monolayer catalyst performance. Examples of Pt/Pd(hkl) and Pt/Au(hkl) will be discussed with in the light of the Pt monolayer morphology. In the last part of the talk we will show the examples of Pb UPD on Pt and Ru modified Au(111) as a prelude to design of the novel 2D catalyst monolayers and nanoclusters configurations. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. [1] SRBrankovic@uh.edu

Effects of Be doping on InP nanowire growth mechanisms

Be-doped InP nanowires were grown by the gold-assisted vapour-liquid-solid mechanism in a gas source molecular beam epitaxy system. The InP nanowire length versus diameter [L(D)] dependence revealed an unexpected transition with increasing Be dopant concentration. At Be dopant concentration below ∼1018 cm−3, nanowires exhibited the usual inverse L(D) relationship, indicating a diffusion-limited growth regime. However, as dopant concentration increased, the nanowire growth rate was suppressed for small diameters, resulting in an unusual L(D) dependence that increased before saturating in height at about 400 nm. The cause of this may be a change in the droplet chemical potential, introducing a barrier to island nucleation. We propose a model accounting for the limitations of diffusion length and monolayer nucleation to explain this behaviour.

Lead Underpotential Deposition on Pt-submonolayer Modified Au(111)

Abstract Lead underpotential deposition on Au(111) surface modified with submonolayer of Pt is studied using cyclic voltammetry and in situ scanning tunneling microscopy methods. The two-dimensional Pt submonolayers (nanoclusters) on Au(111) were obtained by spontaneous Pt deposition on Au(111) from × 103 M {PtCl6}2− + 0.1 M HClO4 solution. The in situ scanning tunneling microscopy data were analyzed using statistical image processing algorithm which enabled quantification of the morphology change on Pt-modified Au(111) surface as a function of applied underpotential. The results suggest that Pb underpotential deposition starts on Au steps and other surface defects, similar to Pb underpotential deposition on Au(111). The further process proceeds by Pb monolayer nucleation and growth on Au terraces into a complete layer. In parallel, the Pb monolayer starts to nucleate on top of the Pt nanoclusters. The final stage of the Pb underpotential deposition is formation of the compact Pb nanoclusters/layer on top of the pre-existing Pt nanoclusters. The scanning tunneling microscopy data suggests that morphology of underpotentially deposited Pb monolayer on Pt-modified Au(111) is similar to the starting surface in terms of the areal density of nanoclusters, their size and shape. The morphological changes of the Pt modified Au(111) surface during Pb underpotential deposition are correlated with cyclic voltammetry results.

The Role of Stable and Mobile Carbon Adspecies in Copper-Promoted Graphene Growth

Early stages of carbon monolayer nucleation on the copper (111) surface are systematically studied using density-functional theory calculations in the context of chemical vapor deposition and irradiation-mediated growth of graphene. By analyzing the kinetics of carbon atoms during their agglomeration, including surface, subsurface, and surface-to-bulk migration as well as dimer formation and diffusion, we draw a qualitative picture of the first stages of graphene growth on copper. The formation and migration of dimers and graphitic fragments happens at a much faster rate than the other competing processes, such as carbon migration into copper bulk and the dissociation of dimers into carbon monomers. To explain this tendency, which is an important factor in making copper such an effective graphene catalyst, we analyze in detail the electronic structure of dimers on surfaces and suggest that dimer stabilization and mobility stem from a delicate interplay between the carbon dimer σp bonding orbitals and copper d and s electrons. Our results emphasize the role of mobile carbon dimer intermediates during the growth of graphene on Cu, Ag, and Au surfaces by chemical vapor deposition and irradiation-mediated methods, in which carbon atoms are implanted into copper foils beyond the solubility limit.

Effect of H 2O chemisorption on passivation of Ge(100) surface studied by scanning tunneling microscopy

The electronic passivation of a Ge(100) surface, via the chemisorption of H 2O at room temperature (RT), and the temperature dependence of H 2O coverage were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). With a saturation H 2O dose at RT, a highly-ordered structure, due to the dissociative chemisorption of H 2O, was observed on a Ge(100) surface with a coverage of 0.85 monolayers (ML). Annealing the room temperature H 2O-dosed Ge surface to 175 °C decreased the coverage of H 2O to 0.6 ML. Further annealing at 250 °C decreased the coverage of H 2O sites to 0.15 ML, and the surface reconstruction of Ge dimers was observed over much of the surface. Annealing above 300 °C induced Ge suboxide structures, similar to the oxygen-dosed Ge surface. STS measurements confirmed that the surface dangling bond states near Fermi energy are removed by the H 2O chemisorption because the dangling bonds of Ge atoms are terminated by ―OH and ―H. The H 2O pre-dose at room temperature provides a template for the ultrathin passivation of Ge(100) surface via atomic layer deposition (ALD) at RT, since near monolayer nucleation can be obtained with a 1/2 hydroxylated and 1/2 hydrogenated Ge surface.

Atomic imaging of the monolayer nucleation and unpinning of a compound semiconductor surface during atomic layer deposition

The reaction of trimethyl aluminum on the group III rich reconstructions of InAs(0 0 1) and In(0.53)Ga(0.47)As(0 0 1) is observed with scanning tunneling microscopy/spectroscopy. At high coverage, a self-terminated ordered overlayer is observed that provides the monolayer nucleation density required for subnanometer thick transistor gate oxide scaling and removes the surface Fermi level pinning that is present on the clean InGaAs surface. Density functional theory simulations confirm that an adsorbate-induced reconstruction is the basis of the monolayer nucleation density and passivation.

Determining the nucleation rate from the dimer growth probability

A new method is proposed for the determination of the stationary one-component nucleation rate J with the help of data for the growth probability P2 of a dimer which is the smallest cluster of the nucleating phase. The method is based on an exact formula relating J and P2, and is readily applicable to computer simulations of nucleation. Using the method, the dependence of J on the supersaturation s is determined by kinetic Monte Carlo simulations of two-dimensional (2D) nucleation of monolayers on the (100) face of Kossel crystal. The change of J over nearly 11 orders of magnitude is followed and it is found that the classical nucleation theory overestimates the simulation J values by an s-dependent factor. The 2D nucleus size evaluated via the nucleation theorem is described satisfactorily by the classical Gibbs-Thomson equation and its corrected version accounting for the spinodal limit of 2D nucleation.

In situ observation of colloidal monolayer nucleation driven by an alternating electric field.

The nucleation of crystalline materials is a hotly debated subject in the physical sciences. Despite the emergence of several theories in recent decades, much confusion still surrounds the dynamic processes of nucleation. This has been due in part to the limitations of existing experimental evidence. Charged colloidal suspensions have been used as experimental model systems for the study of crystal nucleation and structural phase transitions, as their crystallization phase diagram is analogous to that of atomic and molecular systems, but they can be visualized using microscopy. Previously, three-dimensional imaging of colloidal nucleation dynamics was achieved using confocal microscopy. However, the limited temporal resolution of the confocal microscope is of concern when trying to capture real-time colloidal crystal nucleation events. Moreover, as the thermodynamic driving force has remained undefined, data on key factors such as the critical nuclei size are at best semiquantitative. Here we present real-time direct imaging and quantitative measurements of the pre- and post-nucleation processes of colloidal spheres, and the kinetics of nucleation driven by an alternating electric field, under well-defined thermodynamic driving forces. Our imaging approach could facilitate the observation of other rarely observed phenomena, such as defect and grain-boundary formation and the effects of foreign particles during crystallization. Furthermore, it may prove useful in identifying optical and biological technologies based on colloids.

Epitaxial growth of C60 crystals vapor-deposited on a KI (001) surface

Epitaxial growth of C60 was found in vapor-deposited crystals on the KI (001) cleavage surface. C60 molecules deposited on the KI surface, which was preheated at 400 °C, then kept at 195 °C during deposition, crystallized in the face-centered cubic form with two types of morphologies. Truncated pyramidal crystals epitaxially grew along the KI 〈110〉 directions, taking its C60 (001) face parallel to the KI (001) surface. This epitaxial nucleation occurred at a corner of the crossing steps along the KI 〈100〉 directions which were caused by thermal etching of the KI surface. Molecular mechanics and dynamics calculations revealed that a three-dimensional (2×2) commensurate lattice matching between the C60 {100} planes and the KI {100} faces at the step corner attributed to the epitaxial nucleation of the pyramidal crystal, in spite of the lower stability of the monolayer interaction of the C60 (001) face with the KI (001) surface. On the other hand, platelike crystals grew, with the C60 (111) face parallel to the KI (001) surface. A stable monolayer nucleation of the close-packed C60 (111) face made this two-dimensional growth of the platelike crystals possible to occur on the (001) terrace of the etched KI surface.

Dynamics of two-dimensional self-aggregation: pressure and pH-induced structural changes in a fluorocarbon amphiphile at liquid-air interfaces. An x-ray synchrotron study

In order to provide, on the molecular level, information on crystal nucleation of monolayers at air-water interfaces, the self-aggregation of 1H,1H,2H,2H-perfluorododecyl aspartate CF 3 (CF 2 ) 9 (CH 2 ) 2 OCOCH 2 CH(NH 3 + )CO 2 - (PFA) over water subphases at various pH values was studied using synchrotron X-ray grazing incidence diffraction (GID), including Bragg rods (BR) and reflectivity (XR) measurements

Surface growth topography of grain boundaries in Al thin films

Surface growth topography of grain boundary (GB) areas has been studied on aluminium films deposited at 570 K on mica, NaCl and glass substrates at different levels of oxygen contamination. Broad and deep GB grooves decorated by secondary nuclei were found in highly contaminated films, while hills bordered by pinned growth steps were present at very low oxygen contamination in films deposited on NaCl. Hills were observed at all types of [III] tilt boundaries independent on their azimuthal orientations. A model has been proposed for the interpretation of the development of GB grooves and hills. The model was based on the processes of monolayer crystal growth controlled by the purity of GBs. Pure GBs can be active in the nucleation of monolayers growing the crystals, while contaminated GBs are adsorption sites where impurity species are preferentially segregated and incorporated. Transmission electron microscopy of Pt shadowed films was used to study the GB displacements taking place in high purity films.

Crystal growth kinetics of boron oxide under pressure

We have measured the crystal growth rate u of B2O3-I in the amorphous phase, as it varied over five orders of magnitude with changes in temperature and pressure. We eliminated the crystal nucleation barrier by seeding the surface of boron oxide glass with crystals. u became measurable only when the pressure exceeded a threshold level near 10 kbar. Using the published thermodynamic information on the B2O3 system and a crude free-energy model for the crystal and glass phases, we account qualitatively for our results with the theory of crystal growth limited by the rate of two-dimensional nucleation of monolayers. The constants for the prefactor, activation energy, activation volume, and ledge tension are determined by fitting. By adjusting the thermodynamic parameters to a set of values that are well within the ranges delineated by their experimental uncertainties, we account quantitatively for the measured growth rates from 300 to 500 °C and from 0 to 30 kbar with the following relation: u(T,P)=(785 m/s)[‖ΔGm‖/(RT)]1/6 ×exp[π×3 Å(420 erg/cm2)2(28 cm3/mole)/(3 kTΔGm)]exp[−10 366 cal/mole/(RT)] ×exp[−P×16 cm3/mole/(RT)]×{1−exp[ΔGm/(RT)]}2/3, with the driving free energy given by ΔGm(T,P)=(13 cm3/mole) [PM(T)−P] and the melting curve given by PM(T)=(T−450 °C)/(42.6 K/kbar). The ‘‘B2O3 crystallization anomaly’’, that crystals have never been observed to grow at atmospheric pressure, is explained, since according to our model, the frequency of two-dimensional nucleation is negligible at all temperatures at pressures less than 10 kbar.

Nucleation of platinum and iridium monolayer growth on (110) tungsten surfaces

Two modes of Pt and Ir monolayer nucleation are described that are different from those usually envisaged for island growth. Field ion microscopic images are included showing stages in monolayer growth. (JRD)