Fractional Monolayer Research Articles

Wafer-Scale Emission Energy Modulation of Indium Flushed Quantum Dots

Semiconductor self-assembled quantum dots (QDs) have garnered immense attention for their potential in various quantum technologies and photonics applications. Here, we explore a novel approach for fine-tuning the emission wavelength of QDs by building upon the indium flush growth method: Submonolayer variations in the capping thickness reveal a non-monotonic progression, where the emission energy can decrease even though the capping thickness decreases. indium flush, a well-known technique for inducing blue shifts in quantum dot emissions, involves the partial capping of QDs with GaAs followed by a temperature ramp-up. However, our findings reveal that the capping layer roughness, stemming from fractional monolayers during overgrowth, plays a pivotal role in modulating the emission energy of these QDs. We propose increased indium interdiffusion between the QDs and the surrounding GaAs capping layer for a rough surface surrounding the QD as the driving mechanism. This interdiffusion alters the indium content within the QDs, resulting in an additional emission energy shift, counterintuitive to the capping layer’s thickness increase. We utilize photoluminescence spectroscopy to generate wafer maps depicting the emission spectrum of the QDs. Using thickness gradients, we produce systematic variations in the capping layer thickness on 3″ wafers, resulting in modulations of the emission energy of up to 26 meV.

Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys

Energetic processing methods such as hyperthermal implantation hold special promise to achieve the precision synthesis of metastable two-dimensional (2D) materials such as Janus monolayers; however, they require precise control. Here, we report a feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) and apply it to investigate the transformation kinetics of monolayer WS2 crystals into Janus WSSe and WSe2 by implantation of Se clusters with different maximum kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and photoluminescence are used to assess the structure, composition, and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of selenium for different kinetic energies that are regulated with in situ ICCD imaging, ion probe, and spectroscopy diagnostics. First-principles calculations, XPS, and atomic-resolution HAADF STEM imaging are used to understand the intermediate alloy compositions and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS2 transforms through WS2(1-x)Se2x alloys to WSe2 and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired composition, and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.

Mapping cell cortex rheology to tissue rheology and vice versa.

The mechanics of biological tissues mainly proceeds from the cell cortex rheology. A direct, explicit link between cortex rheology and tissue rheology remains lacking, yet would be instrumental in understanding how modulations of cortical mechanics may impact tissue mechanical behavior. Using an ordered geometry built on 3D hexagonal, incompressible cells, we build a mapping relating the cortical rheology to the monolayer tissue rheology. Our approach shows that the tissue low-frequency elastic modulus is proportional to the rest tension of the cortex, as expected from the physics of liquid foams as well as of tensegrity structures. A fractional visco-contractile cortex rheology is predicted to yield a high-frequency fractional visco-elastic monolayer rheology, where such a fractional behavior has been recently observed experimentally at each scale separately. In particular cases, the mapping may be inverted, allowing to derive from a given tissue rheology the underlying cortex rheology. Interestingly, applying the same approach to a 2D hexagonal tiling fails, which suggests that the 2D character of planar cell cortex-based models may be unsuitable to account for realistic monolayer rheologies. We provide quantitative predictions, amenable to experimental tests through standard perturbation assays of cortex constituents, and hope to foster new, challenging mechanical experiments on cell monolayers.

Study of Siloxane Adsorption on Hard Disk Media Surface.

Siloxane adsorption on disk surfaces is investigated as a function of lubricant film thickness and carbon film composition through several analytical technologies including FTIR, XPS, TOF-SIMS, GC-MS, and Candela OSA. Disk parameters that govern the propensity for siloxane adsorption are identified. Lubricant film coverage, taken as the fractional monolayer thickness, is found to be the most significant determinant. Siloxane adsorption decreases with increasing lubricant film coverage. The nitrogen content of the underlying carbon film is also found to be a determinant for submonolayer lubricant films: On the one side, siloxane adsorption on carbon films increases with increasing atom % nitrogen in carbon films. On the other side, the lubricant monolayer film thickness increases with increasing atom % N in carbon films, i.e., lubricant film coverage decreases with increasing atom % N. Thus, increasing atom % N in carbon films leads to increasing siloxane adsorption.

Mid- and long-infrared emission properties of InxGa1−xAsySb1−y quaternary alloy with Type-II InAs/GaSb superlattice distribution

In this work, an InxGa1−xAsySb1−y quaternary alloy with a vertical distribution of type-II InAs/GaSb superlattices is grown in the miscibility gap using a fractional monolayer alloy (FMA) process on a vicinal surface. X-ray diffraction patterns indicate that InxGa1−xAsySb1−y quaternary alloys with high In contents (up to 50%–65%) still maintain pure phases and good crystal quality. Transmission electron microscopy with energy-dispersive spectroscopy is used to confirm the constituent elements of the quaternary alloy. In addition, a significant strain distribution phenomenon parallel to the growth direction (ϵxx) can be observed in the quaternary alloy using strain maps and related strain profiles. The mean strain peak values are close to those of conventional planar superlattices, including InAs/GaSb and InAs/InAsSb. Photoluminescence measurements and k∙p model calculations show that the vertical type-II InAs/GaSb superlattice distribution may generate a unique bandgap in the quaternary alloy by manipulating the FMA process on the vicinal surface. The quaternary alloy components and vertically-distributed superlattice parameters can be regulated, and the quaternary alloy bandgap can cover the mid-to long-wavelength infrared regions. This work offers an effective route to solve the miscibility problem and use neoteric quaternary alloys with carrier emission from the unique bandgap for infrared optoelectronic devices.

X-ray mapping in a scanning transmission electron microscope of InGaAs quantum dots with embedded fractional monolayers of aluminium

We investigate AlGaAs/GaAs superlattices as well as InGaAs/GaAs quantum wells and epitaxial quantum dots (QDs) where during the molecular beam epitaxy of InGaAs QDs the aluminium flux cell was opened briefly to incorporate fractional monolayers of Al into the InGaAs. We show that x-ray mapping with a large collection angle is capable of detecting 0.3–0.4 fractional Al monolayers with a resolution of just under 1 nm.

Estimation of the lateral dimensions of epitaxial submonolayer CdSe/ZnSe quantum dots

CdSe fractional monolayer quantum dots (FMQDs) embedded in a ZnSe matrix were produced by atomic layer epitaxy with a nominal coverage of 0.5 monolayer. They have a thickness of a/2, where a is the strained perpendicular lattice constant of cubic CdSe. Their photoluminescence spectra at low temperature exhibit narrow and intense excitonic emission around 2.759 eV. Based on the experimental excitonic emission energy and applying the factorized-envelope approximation, we have estimated that the lateral dimensions of these FMQDs are around 4–5 nm and their density is cm−2.

Submonolayer epitaxy growth of fractional monolayer CdSe/ZnSe quantum dots

A heterostructure containing several sheets of CdSe fractional monolayer quantum dots with nominal coverages of 0.5 and close to 0.25 of a monolayer (ML) embedded in a ZnSe matrix was successfully grown by a combination of submonolayer epitaxy and molecular beam epitaxy. Due to surface reconstruction properties of cation- and anion-terminated surfaces, one complete Cd–Se alternate source exposure cycle produces a submonolayer saturation coverage of around 0.5 ML of CdSe. To obtain specific coverages lower than 0.5 ML, it is necessary to work with an unsaturated Cd surface, then the control of Cd exposure time becomes crucial. To determine the appropriate Cd exposure times, a careful study of the adsorption process and knowledge of the time to obtain the saturation coverage are required. Four separated sheets of CdSe fractional monolayer quantum dots with coverage Θ ≃ 0.5 ML were deposited in the first section of the heterostructure to analyze the surface reconstruction streaks in the reflection high energy electron diffraction (RHEED) patterns. The analysis performed during the adsorption process to reach a Cd saturated surface (nominal coverage Θ = 0.5 ML) suggests that the large Cd atoms produce strain-induced disorder in the underlying layers during the initial stages of the Cd chemisorption process. The results of the RHEED study were employed to estimate the Cd exposure time for the growth of the last sheet of CdSe fractional monolayer quantum dots with Θ ≃ 0.25 ML. The low temperature photoluminescence spectrum presented two intense and narrow excitonic peaks at energies of 2.750 eV and 2.778 eV, corresponding to the CdSe fractional monolayer quantum dots with nominal coverages of 0.5 ML and ∼0.25 ML, respectively, with narrower linewidth for lower CdSe coverage.

Microsecond Resolved Infrared Spectroelectrochemistry Using Dual Frequency Comb IR Lasers.

A dual infrared frequency comb spectrometer with heterodyne detection has been used to perform time-resolved electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The measurement of the potential dependent desorption of a monolayer of a pyridine derivative (4-dimethylaminopyridine, DMAP) with time resolution as high as 4 μs was achieved without the use of step-scan interferometry. An analysis of the detection limit of the method as a function of both time resolution and measurement coadditions is provided and compared to step-scan experiments of an equivalent system. Dual frequency comb spectroscopy is shown to be highly amenable to time-resolved ATR-SEIRAS. Microsecond resolved spectra can be obtained with high spectral resolution and fractional monolayer detection limits in a total experimental duration that is 2 orders of magnitude less than the equivalent step-scan experiment.

Advanced material system for the design of an intermediate band solar cell: Type-II CdTe quantum dots in a ZnCdSe matrix

We explore CdTe fractional monolayer quantum dots (QDs) in a ZnCdSe host matrix for potential application in an intermediate band solar cell device. Careful consideration has been taken during the initiation of the growth process of QDs by migration enhanced epitaxy, in order to avoid the formation of undesirable interfacial layers that can form due to the lack of common anion between the two materials. A superlattice structure of 100 periods of alternating QD and spacer layers is analyzed by high-resolution X-ray diffraction (XRD) and photoluminescent (PL) spectroscopy. Simple arguments are used following continuum elastic theory to deduce the size of the dots and the strain within the superlattice from XRD data. This is further verified using PL and used in the energy calculations that yield the values of the intermediate band energy. The results suggest that the optimized materials are highly suitable for these high efficiency solar cells.

Fabrication and Characterization of an InAs(Sb)/InxGa1−xAsySb1−y Type‐II Superlattice

Infrared optoelectronic devices based on type‐II superlattice structures of III–V semiconductor materials have progressed significantly during the past decades. Exploring and further expanding the material space is of great significance for the development of infrared applications. Herein, a superlattice structure based on InAs(Sb)/InxGa1−xAsySb1−y grown using a fractional monolayer alloy process to control the superlattice composition and structure is presented. High‐order satellite peaks with a narrow full width at half maximum (31 arcs) in the X‐ray diffraction patterns indicate the good crystal quality of the superlattices. Transmission electron microscopy (TEM) with energy‐dispersive spectroscopy (EDS) analysis validate the fractional monolayer alloy growth method. Photoluminescence measurements and k·p model calculations reveal the superior optical properties of the superlattice (sevenfold higher intensity than that of InAs/GaSb). In addition, the flexible control over the superlattice structure afforded by the method allows miniband engineering to cover the mid‐wavelength and long‐wavelength infrared regions. This work highlights the great potential of InAs(Sb)/InxGa1−xAsySb1−y superlattices in infrared optoelectronic devices.

Design and Investigation of Molybdenum Modified Platinum Surfaces for Modeling of CO Tolerant Electrocatalysts

Mo overlayers were prepared on smooth polycrystalline platinum and platinized platinum electrode surfaces by in situ electrochemical deposition of molybdenum oxide at potential below 500 mV for modeling Mo–Pt electrocatalysts. Correlations were found between the applied potential and the amount of deposited Mo, which never exceeded a monolayer, thus Pt–Mo bonds stabilize the deposited Mo oxide. Electrochemical measurements suggested that Mo deposited from a Mo(VI) solution was reduced to the 4+ oxidation state. In line with the ex situ XPS findings a certain part (20–25%) of the initial Mo layer remained irreversibly adsorbed on the Pt/Pt electrode even after oxidation into the 6+ state at high potentials; this fractional monolayer cannot be dissolved even by prolonged cyclic polarization up to 1000 mV. It has been demonstrated that the irreversibly bound Mo partial monolayer is enough to change significantly the CO poisoning properties of the Pt surface. On this Mo:Pt (1:4) surface CO oxidation is initiated at extremely low potentials (ca. 100 mV). Moreover, only Pt modified by Mo(IV) species is active in low-potential CO oxidation reaction as after oxidizing the irreversibly adsorbed Mo to the 6+ state, CO oxidation is no longer observable. Nevertheless, the catalyst can be reactivated by reduction of molybdenum into the 4+ oxidation state. However, this reduction requires clean, CO-free Pt surface. If Pt is largely covered by CO, reduction of Mo(VI) into Mo(IV) does not occur and thus the low potential CO oxidation remains hindered.

Förster Resonance Energy Transfer and Harvesting in II–VI Fractional Monolayer Structures

We report on Forster resonance energy transfer in the dense arrays of epitaxial quantum dots (QDs), formed by fractional monolayer CdSe insertions within a ZnSe matrix. In such arrays comprising the QDs of different sizes, the energy transfer can take place between the ground levels of small QDs and the excited levels of large radiating QDs, when these states are in resonance. This mechanism provides directional excitation of a limited number of the large QDs possessing the excited levels. It reveals itself by the shrinkage of photoluminescence (PL) bands and the appearance of the narrow single excitonic lines in micro-PL spectra. The strong shortening of characteristic PL decay times in the energy-donating QDs is observed when the distance between them and the energy-accepting QDs decreases. Photoluminescence excitation spectroscopy demonstrates the switching of the dominant energy transfer mechanism at the energy predicted by theoretical modeling of the excitonic levels in the QD arrays. Our results pave the way for engineering of the architecture of excitonic levels in the QD arrays to realize efficient nano-emitters.

Recombination dynamics in heterostructures with two planar arrays of II-VI quantum dots

We present time-resolved photoluminescence studies of epitaxial heterostructures with two arrays of Cd(Zn)Se/ZnSe quantum dots (QDs), which are formed by the successive insertion of CdSe fractional monolayers of different nominal thicknesses into a ZnSe matrix. Our data are suggestive of the appearance of effective channels of the energy transfer from the insertion comprising the array with smaller QDs, emitting at higher energy, towards the array with larger QDs, emitting at lower energy. The effect of dark excitons on characteristic times of radiative recombination is discussed.

Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties

Functionalization of semiconductor surfaces with organic moieties can change the charge distribution, surface dipole, and electric field at the interface. The modified electric field will shift the semiconductor band-edge positions relative to those of a contacting phase. Achieving chemical control over the energetics at semiconductor surfaces promises to provide a means of tuning the band-edge energetics to form optimized junctions with a desired material. Si(111) surfaces functionalized with 3,4,5-trifluorophenylacetylenyl (TFPA) groups were characterized by transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and surface recombination velocity measurements. Mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized by electrochemical methods. Current density versus voltage and Mott–Schottky measurements of Si(111)–MMTFPA electrodes in contact with Hg indicated that the barrier height, Φb, was a function of the fractional monolayer coverage o...

Synthesis, Characterization, and Reactivity of Ethynyl- and Propynyl-Terminated Si(111) Surfaces

Ethynyl- and propynyl-terminated Si(111) surfaces synthesized using a two-step halogenation/alkylation method have been characterized by transmission infrared spectroscopy (TIRS), high-resolution electron energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic-force microscopy (AFM), electrochemical scanning–tunneling microscopy (EC-STM) and measurements of surface recombination velocities (S). For the ethynyl-terminated Si(111) surface, TIRS revealed signals corresponding to ethynyl ≡C–H and C≡C stretching oriented perpendicular to the surface, HREELS revealed a Si–C stretching signal, and XPS data showed the presence of C bound to Si with a fractional monolayer (ML) coverage (Φ) of ΦSi–CCH = 0.63 ± 0.08 ML. The ethynyl-terminated surfaces were also partially terminated by Si–OH groups (ΦSi–OH = 0.35 ± 0.03 ML) with limited formation of Si3+ and Si4+ oxides. For the propynyl-terminated Si(111) surface, TIRS revealed the presence of a (C–H)...

(Amino acid + silica) adsorption thermodynamics: Effects of temperature

A thorough understanding of amino acid adsorption by mineral and oxide surfaces has a major impact on a variety of industrial and biomedical applications. Little information currently exists regarding temperature effects on most of these adsorption processes. Deeper thermodynamic analyses of their multiple temperature adsorption isotherms would aid the interpretation of the interfacial interactions. Low solution concentration adsorption isotherms for glycine, lysine and glutamic acid on a silica adsorbent were generated for T=(291, 298 and 310)K. Data analysis via the Clausius–Clapeyron method yielded the isosteric heat of adsorption as a function of fractional monolayer coverage for each adsorptive. Each amino acid showed an exothermic adsorption response. Glycine and lysine experienced a greater negative effect of increased temperature compared with glutamic acid, indicating a greater number of adsorbed molecules than glutamic acid, with the former undergoing intermolecular clustering within the adsorbed phase. Isosteric heat analyses suggest ionic interactions for lysine and hydrogen bonding for glutamic acid, both weakening with increased coverage. In contrast, initial hydrogen bonding led to ionic bonding for glycine with increasing coverage.

Similarity of Stranski-Krastanow growth of Ge/Si and SiGe/Si (001)

This study investigates the onset of islanding (Stranski-Krastanow transition) in strained pure germanium (Ge) and dilute silicon-germanium (SiGe) alloy layers grown by chemical vapour deposition on Si(001) substrates. Integration of compositional profiles is compared to a novel method for quantification of X-ray maps acquired in cross-sectional scanning transmission electron microscopy, together with simulations of surface segregation of Ge. We show that Si1−xGex alloys for germanium concentrations x ≤ 0.27 grow two-dimensionally and stay flat up to considerable layer thicknesses, while layers with concentrations in the range 0.28 &amp;lt; x ≤ 1 form islands after deposition of ∼3.0/x monolayers (=quarter unit cells in the diamond lattice, ML). The uncertainty in the amount of deposited material for pure Ge is ±(0.2–0.3) ML. Modelling shows that of the amount of germanium deposited, 0.7 ML segregate towards the free surface so that only ∼2.3/x ML are directly incorporated in the layer within a few nanometres, in good agreement with our measurements. For pure Ge (x = 1), this thickness is smaller than most values quoted in the literature, which we attribute to the high sensitivity of our method to fractional monolayer changes in the effective chemical width of such thin layers.

Molecular Flux Dependence of Chemical Patterning by Microcontact Printing

We address the importance of the dynamic molecular ink concentration at a polymer stamp/substrate interface during microcontact displacement or insertion printing. We demonstrate that by controlling molecular flux, we can influence both the molecular-scale order and the rate of molecular exchange of self-assembled monolayers (SAMs) on gold surfaces. Surface depletion of molecular ink at a polymer stamp/substrate interface is driven predominantly by diffusion into the stamp interior; depletion occurs briefly at the substrate by SAM formation, but diffusion of molecules into the bulk of the stamp dominates over practical experimental time scales. As contact time is increased, the interface concentration varies significantly due to diffusion, affecting the quality and coverage of printed films. Controlling interfacial concentration improves printed film reproducibility and the fractional coverage of multicomponent films can be controlled to within a few percent. We first briefly review the important aspects of molecular ink diffusion at a stamp interface and how it relates to experimental duration. We then describe two examples that illustrate control over ink transfer during experiments: the role of contact time on monolayer reproducibility and molecular order, and the fine control of fractional monolayer coverage for the displacement printing of 1-adamantanethiolate SAMs by 1-dodecanethiol.

Theoretical and experimental studies of hydrogen adsorption and desorption on Ir surfaces

We report adsorption and desorption of hydrogen on planar Ir(210) and faceted Ir(210), consisting of nanoscale {311} and (110) facets, by means of temperature programmed desorption (TPD) and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. TPD spectra show that only one H2 peak is seen from planar Ir(210) at all coverages whereas a single H2 peak is observed at around 440 K (F1) at fractional monolayer (ML) coverage and an additional H2 peak appears at around 360 K (F2) at 1 ML coverage on faceted Ir(210), implying structure sensitivity in recombination and desorption of hydrogen on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in recombination and desorption of hydrogen on faceted Ir(210) for average facet sizes of 5-14 nm. Calculations indicate that H prefers to bind at the two-fold short-bridge sites of the Ir surfaces. In addition, we studied the stability of the Ir surfaces in the presence of hydrogen at different H coverages through surface free energy plots as a function of the chemical potential, which is also converted to a temperature scale. Moreover, the calculations revealed the origin of the two TPD peaks of H2 from faceted Ir(210): F1 from desorption of H2 on {311} facets while F2 from desorption of H2 on (110) facets.