Monolayer Thick Films Research Articles

Non-Covalent Free-Standing Monolayer Nanoarchitectonics of Heterocoronene-Based Discotic Liquid Crystal Molecules

With excellent charge carrier mobility, tendency to form long-range assembly with self-healing ability, and remarkable chemical and mechanical stability, highly conjugated discotic molecules constitute an important class of materials for electronic applications. Here, we report formation of the stable free-standing molecular film of heterocoronene-based discotic liquid crystal (DLC) molecules, held solely by supra-molecular non-covalent interactions. The films of monolayer thickness (∼4 nm) were lifted from Langmuir trough directly onto a circular ring of diameter up to 2 mm as well as onto TEM grids. Films remain free-standing up to a macroscopic length scale, as revealed by optical and secondary electron microscopy, presumably due to the synergistic effect of intermolecular π–π interactions and the dispersion forces between the peripheral alkyl chains as well as their interdigitation. The monolayer is more than 97% transparent in the visible range. Further, resistive switching measurements carried out on the monolayer DLC film transferred on a silver substrate revealed electrochemical metallization to be the governing process. The switching time was found to decrease exponentially with voltage indicating nucleation to be a possible rate-limiting step. We anticipate that the method presented here will facilitate utilization of such non-covalent free-standing films in flexible memristor and nano-electromechanical systems.

Light-sensitive monolayer-thick nanocrystal skins of face-down self-oriented colloidal quantum wells.

Colloidal quantum wells (CQWs), a quasi-two-dimensional, atomically-flat sub-family of semiconductor nanocrystals, are well suited to produce excellent devices for photosensing applications thanks to their extraordinarily large absorption cross-sections. In this work, we propose and demonstrate a new class of light-sensitive nanocrystal skins (LS-NS) that employ a monolayer of face-down orientation-controlled self-assembled CQWs as the active absorbing layer in the UV-visible range. This CQW LS-NS platform enables non-conventional photosensing operation that relies on the strong optical absorption of the monolayered assembly of CQWs and the subsequent photogenerated potential build-up across the device, allowing for self-powered operation. Here such self-oriented CQWs reduce the surface roughness in their monolayer-thick film, essential to high device performance. Owing to their ease of fabrication and low cost, these devices hold great promise for large-scale use in semi-transparent photosensing surfaces.

Water Adsorption on Mica Surfaces with Hydrophilicity Tuned by Counterion Types (Na, K, and Cs) and Structural Fluorination.

The stability of adsorbed water films on mineral surfaces has far-reaching implications in the Earth, environmental, and materials sciences. Here, we use the basal plane of phlogopite mica, an atomically smooth surface of a natural mineral, to investigate water film structure and stability as a function of two features that modulate surface hydrophilicity: the type of adsorbed counterions (Na, K, and Cs) and the substitution of structural OH groups by F atoms. We use molecular dynamics simulations combined with in situ high-resolution X-ray reflectivity to examine surface hydration over a range of water loadings, from the adsorption of isolated water molecules to the formation of clusters and films. We identify four regimes characterized by distinct adsorption energetics and different sensitivities to cation type and mineral fluorination: from 0 to 0.5 monolayer film thickness, the hydration of adsorbed ions; from 0.5 to 1 monolayer, the hydration of uncharged regions of the siloxane surface; from 1 to 1.5 monolayer, the attachment of isolated water molecules on the surface of the first monolayer; and for >1.5 monolayer, the formation of an incipient electrical double layer at the mineral-water interface.

Optimization of growth parameters to obtain epitaxial large area growth of molybdenum disulfide using pulsed laser deposition

2D transition metal dichalcogenides (TMDCs) are promising materials for device applications owing to their electronic, optical, and material properties varying with the number of monolayers. Synthesis of large area crystalline TMDC thin films is still challenging with techniques such as exfoliation and chemical vapor growth owing to the uncontrollability of deposition area and high temperature growths with toxic precursors, respectively. Pulsed laser deposition (PLD) is a technique that can overcome these challenges owing to stoichiometric layer by layer growth control by optimizing the growth parameters. In this study, we optimize parameters such as temperature, post-growth annealing, inert gas pressure, and substrate–target distance during PLD growth of MoS2 to obtain uniform and highly crystalline thin films on an ∼1 in.2 substrate. The optimized growth conditions are 800 °C with a 30 min post-growth annealing at a laser fluence of 2.2 J/cm2 with a substrate–target distance of 5 cm and 0.5 mTorr of argon partial pressure. An RMS roughness of 0.17 nm was obtained for 3 nm (4 monolayers) thick MoS2 films with a thin film conductivity of ∼4000 S/m.

Hole-injection barrier across the intermolecular interaction mediated interfacial DNTT layer

The electronic structures and morphology of dinaphthothienothiophene (DNTT) thin films on highly oriented pyrolytic graphite (HOPG) and polycrystalline Au (pc-Au) substrates have been investigated using photoelectron spectroscopy and atomic force microscopy techniques to better correlate the charge-injection barrier with the interfacial structure. DNTT molecules follow Stranski–Krastanov-type growth mode, i.e., highly-dewetted islands/fibers above a relatively-wetted interfacial layer, on both the substrates, but distinctively different electronic structures. On the HOPG surface, the as-grown and thermal-annealed films show a broad and a sharp highest occupied molecular orbital (HOMO) bands, respectively. On the pc-Au surface, a significantly split HOMO band and a reduced hole-injection barrier are observed in a monolayer thick film, for the first time, which are absent in other thick films. Such splitting indicates a presence of a strong intermolecular interaction in the interfacial layer, presumably due to the frontier π-orbital overlapping arising from partially-covered densely-packed herringbone-like arranged molecules. The dewetted-fibers seem to form with up-right oriented molecules having negligible interaction, which desorb easily by thermal annealing to retain the highly-stable interfacial layer. The reduction of the hole-injection barrier at the DNTT/pc-Au interface, due to the formation of such interfacial layer having strong intermolecular interaction, has an immense importance in increasing the charge-injection efficiency from a practical electrode.

Growth of Crystallographically Aligned PbSe Films of Controlled Thickness on Amorphous Substrates

AbstractLead selenide (PbSe) has extensively been investigated due to its thermoelectric and photoconductive properties. More recently, predictions of emergent properties have focused synthetic efforts on preparing ultra‐smooth layers of crystallographically aligned PbSe layers with a thickness equal to an integer number of monolayers. While rough films are typically formed through Volmer‐Weber growth on heated substrates, the data presented herein indicates that a crystallographically aligned, ultrathin layer with a defined number of PbSe layers forms on mostly amorphous Mo−Se and V−Se coated Si substrates during nominally room temperature deposition. Since the films are uniform in thickness, the number of PbSe bilayers formed depends on the amount of Pb (or Se) deposited. If the amount of Pb deposited is close to that required for an integral number of unit cells and there is sufficient Se, Laue oscillations are present in the as‐deposited samples. The Laue oscillations reflect the quality and uniformity of the crystalline PbSe domains. This study demonstrates that the substrate does not need to be crystalline to form uniform, crystallographically aligned, integer monolayer thick PbSe films. These results suggest that tuning the strength of interactions between substrate and growing film is necessary to form smooth, crystallographically aligned layers. The substrate does not need to be crystalline for these interactions to be strong enough to enable growth of crystallographically aligned films.

Thermal Bistability of an Ultrathin Film of Iron(II) Spin-Crossover Molecules Directly Adsorbed on a Metal Surface.

Spin-crossover molecules are very attractive compounds to realize multifunctional spintronic devices. Understanding their properties when deposited on metals is therefore crucial for their future rational implementation as ultrathin films in such devices. Using X-ray absorption spectroscopy, we study the thermal transition of the spin-crossover compound FeII((3,5-(CH3)2Pz)3BH)2 from submonolayer to multilayers on a Cu(111) substrate. We determine how the residual fraction of high spin molecules at low temperature, as well as the bistability range and the temperature of switching, depends on the layer thickness. The most spectacular effect is the clear opening of a 35 ± 9 K thermal hysteresis loop for a 3.0 ± 0.7 monolayers thick film. To better understand the role played by the substrate and the dimensionality on the thermal bistability, we have performed Monte Carlo Arrhenius simulations in the framework of a mechanoelastic model that include a molecule-substrate interaction. This model reproduces well the main features observed experimentally and can predict how the spin-crossover transition is modified by the thickness and the substrate interaction.

Effect of the Thickness of a Nickel Film on the W(110) Surface on the Character of the Adsorption of Nitric Oxide Molecules

The adsorption of nitric oxide NO molecules on the surface of structurally ordered 1, 2, and 3 monolayer thick nickel films deposited on the W(110) surface has been studied by the method of reflection-absorption infrared spectroscopy. The structure of the nickel films of all examined thicknesses correspond to that of the Ni(111) crystal. The IR spectra of the NO molecules adsorbed on a 3-monolayer Ni film correspond to those of NO on the bulk Ni(111) crystal. For thinner films, more pronounced differences are observed for the monolayer film. This is due to the additional effect of the underlying W(110) substrate on the intramolecular NO bond and/or distortion of the (111) structure of the monolayer Ni film compared to bulk Ni(111) because of the potential substrate relief and the change of the adsorption geometry and the electron state of the NO molecules.

The role of defects in solid state dewetting of ultrathin Ag film on Si(557)

We have studied the solid state dewetting of ten monolayers thick Ag film deposited on periodically patterned Si(557) surface. The annealing of the system in the ultra-high vacuum at the temperatures between 300 and 400 °C resulted in full agglomeration of the film and formation of faceted single crystalline Ag nanoparticles exhibiting bimodal size distribution. We demonstrated that some particles contain screw dislocations producing a step on the upper particle facet. We related the bimodality in particles distribution with the ability of dislocation-containing Ag particles to evolve by Ostwald ripening mechanism.

On the Promotion of Catalytic Reactions by Surface Acoustic Waves.

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified. Using stroboscopic X‐ray photoelectron spectro‐microscopy, we were able to evidence a sub‐nanosecond work function change during propagation of 500 MHz SAWs on a 9 nm thick platinum film. We quantify the work function change to 455 μeV. Such a small variation rules out that electronic effects due to elastic deformation (strain) play a major role in the SAW‐induced promotion of catalysis. In a second set of experiments, SAW‐induced intermixing of a five monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be a major cause for catalytic promotion.

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.

Chemical Evolution of Pt-Zn Nanoalloys Dressed in Oleylamine.

We report on the shape, composition (from Pt95Zn5 to Pt77Zn23), and surface chemistry of Pt-Zn nanoparticles obtained by reduction of precursors M2+(acac)2- (M2+: Pt2+ and Zn2+) in oleylamine, which serves as both solvent and ligand. We show first that the addition of phenyl ether or benzyl ether determines the composition and shape of the nanoparticles, which point to an adsorbate-controlled synthesis. The organic (ligand)/inorganic (nanoparticles) interface is characterized on the structural and chemical level. We observe that the particles, after washing with ethanol, are coated with oleylamine and the oxidation products of the latter, namely, an aldimine and a nitrile. After exposure to air, the particles oxidize, covering themselves with a few monolayer thick ZnO film, which is certainly discontinuous when the particles are low in zinc. Pt-Zn particles are unstable and prone to losing Zn. We have strong indications that the driving force is the preferential oxidation of the less noble metal. Finally, we show that adsorption of CO on the surface of nanoparticles modifies the oxidation state of amine ligands and attribute it to the displacement of hydrogen adsorbed on Pt. All the structural and chemical information provided by the combination of electron microscopy and X-ray photoelectron spectroscopy allows us to give a fairly accurate picture of the surface of nanoparticles and to better understand why Pt-Zn alloys are efficient in certain electrocatalytic reactions such as the oxidation of methanol.

Evolution of the surface states during the in situ SiN layer formation on AlN/GaN heterostructures

The effect of a monolayer thick SiN film on the surface states of the AlN/GaN heterostructure grown by molecular beam epitaxy was investigated. It was revealed that the submonolayer SiN coating of the AlN surface leads to the formation of an ordered (√3 × √3)R30° structure. Further SiN film growth results in a Si-enriched SiN amorphous film formation. The Fermi level, pinned 1 eV above the valence band maximum of as-grown AlN, jumps 2.3 eV when (√3 × √3)R30° is formed, and then gradually goes to 3.1 eV with an increase in the thickness of the SiN film up to four monolayers. This is explained by the evolution of surface states, as well as the appearance of donor-like states in a Si-enriched SiN film. The presence of donor-like states was confirmed when studying the effect of current collapse in enhancement-mode high electron mobility transistors made of SiN/AlN/GaN heterostructures.

Влияние толщины пленки никеля на поверхности W(110) на характер адсорбции молекул оксида азота

Adsorption of nitic oxide molecules NO on the surface of structurally ordered 1, 2 and 3 monolayer thick nickel films deposited onto W(110) surface has been studied by means of reflection-absorption infrared spectroscopy. The structure of nickel films at all thicknesses studied corresponds to Ni(111) crystal. The IR spectra of NO molecules adsorbed on a 3 monolayer Ni film correspond to those of NO on bulk Ni(111) crystal. At lower film thickness there are differences more pronounced for a monolayer film. This is due to additional effect of underlying W(110) support upon the intramolecular NO bond, and/or to some distortion of (111) structure of monolayer film, compared to bulk Ni(111), because of potential relief of the substrate and, consequently, change of adsorption geometry and electron state of NO molecules.

C60 capping of metallic 2D Tl-Au compound with preservation of its basic properties at the buried interface

So-called metal-induced silicon reconstructions (i.e., metal films of monolayer or submonolayer thickness epitaxially grown on single-crystal silicon substrates in ultra-high vacuum) represent a specific class of low-dimensional advanced materials with potential prospects for electronic and spintronic applications. However, they are highly vulnerable to air and, thus, require protective capping. Finding a suitable material is a challenging task, since, in general, the metal-induced reconstructions are vulnerable also to overgrowth of solid layers. In the present study, we have found that C60 fullerite film shows up as a proper capping layer for the (Tl, Au)/Si(111)7 × 7 compound reconstruction. Due to a perfect non-distractive epitaxial C60 overgrowth, the metallic Tl-Au compound preserves at the deeply buried interface its atomic structure and all basic electronic properties, including spin-splitting of surface-state bands and conductivity of metallic type with a weak antilocalization effect.

The monolayer film thickness as a function of molecular weight for perfluoropolyethers based on the n-perfluorobutylene oxide monomer unit

Abstract “C4-4OH” represents a new family of liquid, low molecular weight perfluoropolyether (PFPE) boundary lubricants based on the n-perfluorobutylene oxide monomer unit, -[(CF2)nO]- where n = 4, terminated by two hydroxyl groups (“Tetraol”) on each chain end. C4-4OH, PFPEs with n = 1.5–3 have been well-utilized as boundary lubricants in technological applications. Here the monolayer film thicknesses for C4-4OH as a function of molecular weight on rigid magnetic media are quantified using three metrological techniques including contact angle goniometry, film burnishing and ellipsometric imaging to corroborate the monolayer film thicknesses. The molecular weight dependence of the monolayer film thickness reveals that the adsorbed film structure of C4-4OH scales with its radius of gyration at the full monolayer film thickness.

Weak Antilocalization at the Atomic-Scale Limit of Metal Film Thickness.

Creation of the 2D metallic layers with the thickness as small as a few atomic layers and investigation of their properties are interesting and challenging tasks of the modern condensed-matter physics. One of the possible ways to grow such layers resides in the synthesis of the so-called metal-induced reconstructions on silicon (i.e., silicon substrates covered with ordered metal films of monolayer or submonolayer thickness). The 2D Au-Tl compound on Si(111) surface having [Formula: see text] periodicity belongs to the family of the reconstructions incorporating heavy-metal atoms with a strong spin-orbit coupling (SOC). In such systems, strong SOC results in the spin-splitting of surface-state bands due to the Rashba effect, the occurrence of which was experimentally proved. Another remarkable consequence of a strong SOC that manifests itself in the transport properties is a weak antilocalization (WAL) effect, which has never been explored in the metal layers of atomic thickness. In the present study, the transport and magnetotransport properties of the 2D Au-Tl compound on Si(111) surface were investigated at low temperatures down to ∼2.0 K. The compound was proved to show behavior of the 2D nearly free electron gas system with metallic conduction, as indicated by Ioffe-Regel criterion. It demonstrates the WAL effect which is interpreted in the framework of Hikami-Larkin-Nagaoka theory, and possible mechanisms of the electron decoherence are discussed. Bearing in mind that besides the (Au, Tl)/Si(111)[Formula: see text] system, there are many other ordered atomic-layer metal films on silicon differing by composition, structure, strength of SOC, and spin texture, which provide a promising area for prospective investigations of the WAL effect at the atomic-scale limit when the film thickness is less than the electron wavelength.

Ultra‐High Vacuum Deposition of Pyrene Molecules on Metal Surfaces

Interfacial properties of organic adsorbates featuring aromatic π‐orbitals on metal surfaces play an important role for organic electronics and spintronics. Pyrene is a flat aromatic molecule with a size between ultimately small benzene and extended graphene segments. The deposition of pyrene molecules onto clean and reactive surfaces with a sub‐monolayer coverage under ultra‐high vacuum (UHV) conditions is challenging, since pyrene is a solid with a high vapor pressure. Here, a sublimation procedure under UHV and image pyrene adlayers on in situ prepared Au(111) and Fe/W(110) substrates by means of low‐temperature scanning tunneling microscopy is presented. For Au(111), the molecule–surface interaction is weak as indicated by the specific herringbone reconstruction of the Au(111) surface that is visible through the self‐assembled pyrene adlayer. Pyrene desorption due to weak intermolecular interaction self‐limits the growth to one monolayer (ML). On the more reactive 2–4 ML thick Fe films on W(110), the molecular order of the pyrene adlayer sensitively depends on the Fe thickness‐dependent dislocation pattern at the substrate surface. Irregular arrangements occur for 1 ML Fe and near substrate dislocations for 2–4 ML Fe. Self‐assembled ordered arrays form predominantly for 2 ML Fe, where the dislocation pattern leaves sufficiently large unperturbed areas between the dislocation lines.

KPFM and XPS Metrology of Self-Assembled Monolayer Barrier for Copper Metallization

Silane based Self assembled monolayer (SAM) has been demonstrated as a diffusion barrier layer on lowK dielectrics for sub 10 nm Cu interconnects for Integrated Circuit (IC) applications. A key problem in developing barrier layer is its monitoring capability, especially its thickness, uniformity, its coverage and defect density. The aim of this work is to develop surface analysis methods for characterizing those variables providing in line monitoring techniques that can predict barrier quality and reliability. The approach presented in this paper was to examine the defect density and coverage properties of the SAM on LowK dielectric using physical methods for the characterization of monolayer thick films, i.e. Variable Angle X-Ray Photoelectron Spectroscopy (VA -XPS) and Kelvin Probe scanning Force Microscopy (KPFM). The SAMs were deposited on three different substrates: bare-Si wafers, thermal oxide/Si and Low-k dielectrics on silicon wafers. The silanization was performed by chemical deposition from various solvents, both polar and non-polar, containing 1-2% silane in ethanol as a solvent at temperature range between 60oC to 120oC. The deposited films where characterized as follows: Variable Angle XPS measurement of High Resolution Multiplex . Next, the XPS data was analyzed as follows: a plot of ln [\U0001d475\U0001d494\U0001d496\U0001d483 \U0001d740\U0001d494\U0001d496\U0001d483 \U0001d470\U0001d490)/(\U0001d475\U0001d490 \U0001d740\U0001d490 \U0001d470\U0001d494\U0001d496\U0001d483+1] Versus 1/sin(α) was drawn , where Nsub is the atomic number density for the substrate , No is the atomic number density for the overlayer, \U0001d740\U0001d494\U0001d496\U0001d483 is the substrate Inelastic Mean free Path, \U0001d740o is the overlayer Inelastic Mean free Path, Isub is the signal intensity of electrons coming from the substrate, Io is the signal intensity of electrons coming from the overlayer and α is the take of angle from the sample surface. The Thickness of the overlayer was calculated from the slope of the graph. Using that data thickness of the SAM was obtained. The method and results will be shown and compared to other methods such as TEM. The XPS data yielded average values on large areal spots (> 10-4 cm2). To achieve high resolution analysis a Kelvin probe force microscopy (KPFM) were performed on SAM. KPFM gives a good image of SAM on low K material. We can learn a lot about Homogeneity and Coverage properties. A study of the nucleation and growth of the SAM, dependent on temperature and time was conducted. It seems that after 60 minutes, the SAM had the best homogeneity and surface coverage. (Figure 1). After mastering the use of the two methods for SAM, the results are compared in order to find the most suitable method for SAM metrology and monitoring that can be employed by the Semiconductor industry. Figure 1

(Invited) Use of E-ALD by SLRR of Cu UPD Layers for the Growth of Pd Thin Films in One-Cell Configuration

This work illustrates the application of electrochemical atomic layer deposition (E-ALD) by surface limited redox replacement (SLRR) in one-cell configuration for the growth of Pd thin films using underpotentially deposited Cu (CuUPD) as a sacrificial metal. The growth of Pd films on Au was monitored by open circuit chronopotentiometry while the roughness evolution and thickness of accordingly deposited Pd films were assessed by HUPD and CuUPD cyclic voltammetry and stripping experiments, respectively. The growth of smooth Pd films was observed for about 15 equivalent monolayers (MLs) of Pd followed by a rapid transition to dendritic growth. A comparison with counterpart results of Pd SLRR deposition in a flow-cell suggests that the dendritic growth in one-cell is associated with establishment of mass-transport limitations. To address this shortcoming a modified protocol introducing SLRR-cycle disruption and forced convection was applied to demonstrate the growth of 26 MLs thick, smooth and continuous Pd film.