Surfactant-mediated Growth Research Articles

Electrochemical Epitaxy: Perspective and Applications

Properties of heteroepitaxial thin films are often dependent on the method and conditions for deposition[1]. In that respect, the electrochemical growth represents energetically different approach as compared to conventional vacuum deposition methods. It offers unique advantages, which can be exploited to get high quality ultra-thin films. The obvious one is that process occurs at an ambient temperature, which allows growth of heteroepitaxial metal overlayers while preserving the integrity of the interface. In recent years, significant progress has been made in controlling the thin film growth modes[2],[3],[4]. Various approaches to manipulate growth kinetics and enhance the evolution of atomically flat epitaxial overlayers were discovered[5],[6],[7]. Some of these findings were successfully implemented in electrochemical epitaxial growth resulting in development of deposition protocols such as Defect Mediated Growth (DMG)[8],[9], Surfactant Mediated Growth (SMG)[10] and growth via Surface Limited Redox Replacement (SLRR)[11],[12] with its sub-derivatives such as SEBALD, ECALD, ECALE, E-less ALD. These methods are used extensively by practitioners in different areas to synthesize monolayer or nanocluster catalysts and ultra-thin films with different compositions and applications.In this talk, we will review current perspective and progress on electrochemical epitaxial growth. Discussion will focus on fundamental and practical details of the relevant phenomena about UPD, SMG, DMG, SLRR and E-less ALD using examples and challenges that are facing researchers in these areas. If time allowed, some out of the box idea will be posted opening podium for discussion.

Recent Advances In Template Assisted Growth Engineering of Inorganic Nanocrystals

Nanoparticles (NPs) of controlled size, shape, and composition have attracted significant attention due to their applicability in a future generation device. The interaction of particles with interfaces varies with the change in their morphology. The anisotropic nanostructures are of considerable attention for scientific and technological application as they are expected to exhibit substantial advantages over their isotropic counterparts in several fields, such as enhanced energy harvesting, improved charge transport properties, nonlinear optical responses, enhanced optical gain, and polarized light absorption and emission. It is noteworthy to mention that the crystallization from a solution governs the growth process which subsequently determines the shape of the particle. Several competing factors, such as preferential attachment to different crystal facets, surfactant-mediated directional growth, and the utilization of surface coordinating ligands, induce the morphologies of anisotropic particles. A number of fascinating anisotropic nanostructures, including nanorods, wires, tetrapods, multipods, dumbbells, and platelets, have been synthesized employing solution chemistry. However, still there is a quest of low cost and easily scalable synthesis strategies to systematically develop size and shape modulated nanoparticles. In this review, the state-of-the-art in the area of the synthesis techniques for the exploration of size and shape control of a variety of inorganic nanomaterials is overviewed. The crystal growth mechanisms both intrinsically and in the presence of templates are discussed. Finally, we conclude with the recent development in the research progress in this topic as well as our perspectives on the challenges and opportunities.

Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC.

We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely R0° rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a preoriented template to induce the unconventional orientation. Using spot profile analysis low-energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.

Surfactant-Mediated Growth and Patterning of Atomically Thin Transition Metal Dichalcogenides.

The role of additives in facilitating the growth of conventional semiconducting thin films is well-established. Apparently, their presence is also decisive in the growth of two-dimensional transition metal dichalcogenides (TMDs), yet their role remains ambiguous. In this work, we show that the use of sodium bromide enables synthesis of TMD monolayers via a surfactant-mediated growth mechanism, without introducing liquefaction of metal oxide precursors. We discovered that sodium ions provided by sodium bromide chemically passivate edges of growing molybdenum disulfide crystals, relaxing in-plane strains to suppress 3D islanding and promote monolayer growth. To exploit this growth model, molybdenum disulfide monolayers were directly grown into desired patterns using predeposited sodium bromide as a removable template. The surfactant-mediated growth not only extends the families of metal oxide precursors but also offers a way for lithography-free patterning of TMD monolayers on various surfaces to facilitate fabrication of atomically thin electronic devices.

Beyond‑carbon-solvency effects of catalytic metal Ni on diamond growth

To understand the physical and chemical roles of catalytic metal Ni in the growth of diamond, ab-initio calculations of the structural, electronic, and kinetic properties of a Ni-covered C (111) surface were performed. Findings from this theoretical study highlight two important roles of Ni in addition to its carbon-solvency effect, widely known to play a catalytic role in the growth of diamond. The first role is to facilitate the formation of a thermodynamically stable Ni-C interface with a diamond bulk-like structure and the second is to induce surfactant-mediated growth enabling continuous layer-by-layer growth for diamond.

(Electrodeposition Division Research Award Address) Metal Deposition Assisted by UPD Monolayers-Fundamentals and Applications

The improved understanding of nucleation and growth kinetics provided the base for design of new experimental approaches where the thin film growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. Following this progress, a new protocols for electrochemical thin film growth were developed which benefit from the Underpotentially Deposited Monolayers (UPD ML) serving as a surfactants (Surfactant Mediated Growth, SMG), mediators (Defect Mediated Growth, DMG) or sacrificial materials (Deposition via Surface Limited Redox Replacement, SLRR) in electrodeposition process of thin films. In these examples, the essential step that results in desired 2D growth mode is achieved by precise control of the electrode potential, electrodeposition sequence and/or solution chemistry which enable particular action/benefit of the UPD ML to be fully exploited in the growth process. In the first part of the talk we will briefly review fundamentals of the electrochemical thin film growth assisted by UPD ML. Our results and discussion are focused on examining the extendibility of SMG, DMG and SLRR approach for thin film growth in technologically relevant system such as Cu/Ru(hklm). The Cu/Ru(hklm) represents the overlayer/substrate system which has significant epitaxial strain (-5.5%). This fact together with room temperature growth conditions favor a 3D growth. However, as it will be shown, with convenient use of UPD ML serving as surfactant, or mediator, or sacrificial material a 2D growth of Cu on Ru can be achieved readily. In the second part of the talk we will demonstrate that UPD-like ML can be deposited on different substrates using an electroless (e-less) approach. The example of improved morphology and magnetic properties of e-lessly deposited Co overlayers on Cu assisted by e-lessly deposited Pb ML serving as surfactant are analyzed. The phenomenon of e-less UPD-like ML deposition as a prelude for e-less atomic layer epitaxial deposition (E-less ALD) will be discussed as well. We will show results demonstrating noble metal thin film growth on Cu and other substrates using e-lessly deposited Pb UPD-like ML serving as a reducing precursor in SLRR reaction with noble metal ions such as Pt, Pd, and Ru. In conclusion of the talk, (if time permitted), the results from exploratory research studying the benefit of UPD ML serving as a stress moderators in electrodeposition process of Pd thin films will be discussed. The author acknowledges support from Lam Research Corporation and NSF Chemistry division under the contract # 0955922 and #1605331.

Surfactant Mediated Growth of Cu on Ru(0001)

Electrodeposition represents the ambient temperature process offering the opportunity to grow many heteroepitaxial metal overlayers while preserving the integrity of their interface. In recent years, significant progress has been made in understanding the kinetic aspects controlling the growth modes in electrodeposition [[i],[ii],[iii]]. The improved understanding has led to various discoveries in which the growth kinetics was manipulated to enhance the evolution of atomically flat epitaxial overlayers [[iv],[v],[vi]]. Exploiting some of these results has led to development of several new methods and protocols for electrodeposition of heteroepitaxial thin films assisted by UPD monolayer serving as a mediator, surfactant or sacrificial template. [[vii],[viii],[ix],[x]]. In this talk we will briefly review the progress in the electrochemical thin film growth assisted by UPD ML. Our focus is examining the epitaxial growth of Cu on Ru(0001) assisted by Pb UPD ML serving as a surfactant. Cu and Ru represent the overlayer/substrate system which has significant epitaxial strain (-5.5%). This fact together with room temperature growth conditions favor a 3D growth. However, (Figure 1), with convenient use of Pb UPD ML serving as surfactant, a 2D growth of Cu on Ru is induced. In situ STM study indicates that Pb significantly enhances mobility of Cu thus improving inter- and intra-layer transport which leads to evolution of flat epitaxial Cu deposit. An uncommon for Cu growth at room temperature, a surface evolution was characterized by step flow indicating significantly reduced barrier for adatom step-down process. XPS study indicates that amount of Pb in Cu deposit is a function of the solution design and experiment control. Characterization of 2D Cu overlayers indicates that their quality is comparable to MBE grown films which qualifies our approach as relevant for many high-tech applications. Our results also re-confirm the fact that Pb UPD ML-assisted electrochemical growth could be used to produce smooth 2D films of various metal overlayers even in situations where both thermodynamics and kinetics conditions do not favor positive outcome. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. Figure 1. Insitu STM images obtaned during Cu growth on Ru(0001) from 10-2 M Pb2+ + 5x10-5 M Cu2+ + 0.05 M HClO4 solution at E=-200 mV vs. Ag+/AgCl. [i] Physics of Crystal Growth, A. Pipinelli and J. Villain, University Press, Cambridge, UK (1998). [ii] J. Tersoff, A.W. Denier van der Gon, and R.M. Tromp, Phys. Rev. Lett. 72, 266 (1994). [iii] G. Rosenfield, B. Polesma and G. Comsa in Chemical Physics of Solid Surfaces ed. D. A. King and D. P. Woodruff, vol. 8, Elsevier, New York (1997), p. 70. [iv] G. Rosenfeld, R. Servaty, C. Teichert, B. Poeselma and G. Comsa, Phys. Rev. Lett., 71, 895 (1993). [v] H. A. van der Vegt, H.M. van Pinxteren, M. Lohmeier, E. Vlieg and J.M.C. Thornton, Phys. Rev. Lett. 68, 3335 (1992). [vi] H. A. van der Vegt, H.M. van Pinxteren, M. Lohmeier, E. Vlieg and J.M.C. Thornton, Phys. Rev. Lett. 68, 3335 (1992). [vii] K. Sieradzki, S.R. Brankovic and N. Dimitrov, Science, 284, 138 (1999). [viii] S. R. Brankovic, Electrochemical Mediated Growth, PhD Thesis, Arizona State University, Tempe (1999) [ix] S.R. Brankovic, N. Dimitrov and K. Sieradzki, Electrochem.Solid-State Lett., 2, 443 (1999). [x] S.R. Brankovic, J.X. Wang and R. R. Adzic, Surf. Sci., 474, L173 (2001). Figure 1

Tracing the Surfactant-Mediated Nucleation, Growth, and Superpacking of Gold Supercrystals Using Time and Spatially Resolved X-ray Scattering.

The nucleation and growth process of gold supercrystals in a surfactant diffusion approach is followed by simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS), supplemented with scanning electron microscopy. The results indicate that supercrystal nucleation can be activated efficiently upon placing a concentrated surfactant solution of a nematic phase on top of a gold nanocrystal solution droplet trapped in the middle of a vertically oriented capillary tube. Supercrystal nuclei comprised of tens of gold nanocubes are observed nearly instantaneously in the broadened liquid-liquid interface zone of a steep gradient of surfactant concentration, revealing a diffusion-kinetics-controlled nucleation process. Once formed, the nuclei can sediment into the naoncrystal zone below, and grow efficiently into cubic or tetragonal supercrystals of ∼1 μm size within ∼100 min. Supercrystals matured during sedimentation in the capillary can accumulate and face-to-face align at the bottom liquid-air interface of the nanocrystal droplet. This is followed by superpacking of the supercrystals into highly oriented hierarchical sheets, with a huge number of gold nanocubes aligned for largely coherent crystallographic orientations.

Surfactant-Mediated Growth of Nanostructured MoBiInS5 Thin Films via Arrested Precipitation Technique

Surfactant-Mediated Growth of Nanostructured MoBiInS₅ Thin Films via Arrested Precipitation Technique

Growth of L10–FeNi thin films on Cu(001) single crystal substrates using oxygen and gold surfactants

We alternately deposited Ni and Fe monatomic layers on Cu(001) single crystal substrates and also performed surfactant-mediated growth. FeNi films exhibited a large uniaxial magnetic anisotropy (Ku) of over 0.1MJ/m3 by using an oxygen or a gold surfactant, although Ku was almost zero without surfactants. Reflection high-energy electron diffraction and atomic force microscopy indicated surfactant effects of oxygen and gold. Synchrotron X-ray diffraction indicated a formation of an L10-type ordered FeNi phase having a large Ku. We have considered that the surfactants enhanced layer-by-layer growth leading to the L10-ordering by changing a diffusion scenario of Fe and Ni atoms.

Optical in situ calibration of Sb for growing disordered GaInP by MOVPE

Reflectance Anisotropy Spectroscopy (RAS) was employed to determine the optimal specific molar flow of Sb needed to grow GaInP with a given order parameter by MOVPE. The RAS signature of GaInP surfaces exposed to different Sb/P molar flow ratios were recorded, and the RAS peak at 3.02eV provided a feature that was sensitive to the amount of Sb on the surface. The range of Sb/P ratios over which Sb acts as a surfactant was determined using the RA intensity of this peak, and different GaInP layers were grown using different Sb/P ratios. The order parameter of the resulting layers was measured by PL at 20K. This procedure may be extensible to the calibration of surfactant-mediated growth of other materials exhibiting characteristic RAS signatures.

Kinetic Monte Carlo Simulation of Surfactant-Mediated Thin Film Growth

A novel model consisting of basic micro-processes has been developed on the basis of the classic diffusion theory. It is first time that the concept of exchange rate has introduced，and the growing process of surfactant-mediated epitaxial thin-film growth has been simulated with Kinetic Monte Carlo (KMC) technique. The results of simulation found that the exchange reaction of RLA model is a combination of the basic micro-processes. The majorities of exchange are not complete position exchange and the exchange rate doesn’t always equal one. Both surfactant atoms and adatoms would diffuse from one layer to another. The diffusion happens mostly between single atoms and the diffusing atoms increase with substrate temperature.

Growth mode control of InGaN on GaN by using SiH4 post-treatment

We report on a way to control the growth mode of an In0.43Ga0.57N layer on GaN by using SiH4 post-treatment. In0.43Ga0.57N islands were formed on the GaN layers by a using Stranski-Krastanov growth mode without SiH4 post-treatment. When the SiH4 flow rate was increased from 0 to 10 sccm, the InGaN island density decreased from 3.3 × 109 to 1.9 × 109 then to zero, resulting in two-dimensional growth. However, a further increase in the SiH4 flow rate to 15 sccm again led to the formation of InGaN islands. Temperature- and excitation-power-dependent photoluminescence spectra showed that the InGaN islands provided strong localized recombination centers for electrical charge carriers and that the InGaN layers were composed of InGaN islands with different numbers of monolayers. This abnormal growth behavior with SiH4 post-treatment was explained by the combined effects of surfactant-mediated growth modification and the formation of Si x N y with the injection of SiH4.

Growth of ZnO hemispheres on silicon by a hydrothermal method

This paper reports a hydrothermal method to control the morphology of hemisphere ZnO nanostructures (NSs) fabricated on a Si substrate by using surfactant additive molecules. The shape of ZnO NSs was controlled by the concentration of the trisodium citrate (TSC) added during the main growth step. With increasing the TSC concentration from 0 to 1.3 and 2.6mM, the shape of ZnO NSs changed from a bundle of nanorods to a flower-like-shape composed of merged large hexagonal crystals, and finally to ZnO hemispheres. X-ray diffraction and photoluminescence spectroscopy revealed that the ZnO NSs had a wurtzite crystalline structure. The shape evolution mechanism of ZnO NSs could be explained using the surfactant-mediated growth model.

Enhanced Exchange Coupling in TiO$_{2}$-Doped FePt-C Nanocomposite Thin Films Induced by Surfactant Mediated Growth

We reported the TiO2 doping effects on the microstructure and magnetic properties of FePt-C nanocomposite films (dual doping of C and TiO2). X-ray diffraction measurements revealed that the (001) orientation was well kept and there was no observable deterioration in chemical ordering degree, indicating a negligible change in anisotropy energy with increased TiO2 doping. The coercivity of FePt-C-TiO2 films decreased, while the slope of the hysteresis loops increased, with TiO2 volume fraction, implying an enhanced exchange coupling induced by TiO2 doping. Angular-dependent switching fields of FePt-C-TiO2 films depicted that the reversal mode was changed from coherent-rotation dominated to wall-motion dominated, possibly resulting from the enhanced exchange coupling. Cross-sectional transmission electron microscope images showed that the grain boundaries became less clear and grains either coalesced or contacted each other at increased TiO2 doping. The compositional-depth profile revealed that the C was pushed onto the film surface due to TiO2 doping. A surfactant mediated growth model was proposed to account for the observed enhancement of exchange coupling and the change of microstructure. This study showed that to reduce grain size and promote grain isolation, a proper selection of boundary materials with different surface free energy is needed.

In situobservation of stress and strain evolution during surfactant-mediated growth of Ge on Si

<i>In situ</i>observation of stress and strain evolution during surfactant-mediated growth of Ge on Si

A Comparative Study of Au Grown by Pb UPD Assisted Methods (Surface Limited Redox Replacement, Surfactant Mediated Growth) and Overpotential Deposition

not Available.

Effect of antimony irradiation on InAs/Sb:GaAs quantum dots grown by molecular beam epitaxy

In this paper we discuss the fabrication of high-density InAs quantum dots (QDs) on GaAs (1 0 0) substrates by molecular beam epitaxy, via antimony surfactant-mediated growth. The structural and optical properties of the QDs were studied as a function of the amount of antimony irradiation. It was found that with appropriate antimony irradiation, the dot density significantly increases while simultaneously restraining giant QDs. However, when the irradiation is beyond a certain amount, the dot density inversely decreases significantly. Moreover, temperature-dependent photoluminescence (T-PL) measurements reveal that antimony irradiation can cause a larger redshift of the PL peak wavelength at all temperatures, a faster decrease of integrated PL intensity below 100 K and a slower decrease of integrated PL intensity within 100–300 K as compared to measurements taken in the absence of antimony irradiation. It was also found that by increasing the amount of antimony irradiation, the samples' behavior is closer to that of conventional QDs.

Surfactant Mediated Growth of Cu and Ag Thin Films

not Available.

Optical and electrical properties of stacked binary InAs-GaAs quantum dot structures prepared under Surfactant-mediated growth conditions

The structural, optical and electrical properties of a 10-layer InAs/GaAs quantum dots (QDs) system having InAs layers (2.9ML) grown under surfactant growth conditions, using only an impinging In beam, were investigated. This growth mode still resulted in the formation of quantum dots, but with dot sizes smaller and sample quality better than those for normal growth (NG) of ~3ML InAs-GaAs QD structures. Room temperature photoluminescence measurements showed PL emission from this sample at 1200-1300 nm, i.e. reaching the telecom O-band. At low substrate growth temperatures (LT), 250°C, and under the same "Arsenic free" growth condition an InAs/GaAs superlattice structure without the formation of QD was successfully grown with up to 2.9MLs of InAs, which was not achievable under NG conditions. Both samples showed a noticeable photocurrent when illuminated with 1.2 – 1.3 μm lasers. Thus they can be used as photoconductive materials that can be excited with wavelength longer than that used for the well-known LT-GaAs ultrafast material and close to the telecom wavelengths of 1.3 μm for Terahertz imaging or other optoelectronic applications.