Nanometer-thick Films Research Articles

Strain-Induced Photocurrent Enhancement in Photodetectors Based on Nanometer-Thick ZnO Films on Flexible Polydimethylsiloxane Substrates

Strain-induced modulation of electronic, magnetic, and photonic properties provides additional functionalities to oxide thin film-based electronics as compared to those of conventional rigid electr...

Combining Sub-nanometer Adhesion and Capping Layers for Thermally Stable Nanometer-Thick Au Films

Improving the thermal stability of Au thin films is critical if thermo-plasmonic applications such as heat-assisted magnetic recording are to become commercially viable. In this work, Al capping la...

Integration of Nanometer-Thick 1T-TaS2 Films with Silicon for an Optically Driven Wide-Band Terahertz Modulator

The amplitude of terahertz (THz) waves is modulated optically by a pumping laser source, and the effect of optical power on modulation depth is systematically investigated in this work. The reported THz modulator is based on a conducting transition metal dichalcogenide (TMD), that is, a nanometer-thick thin film of tantalum disulfide (TaS2) grown on a high-resistivity silicon (Si) substrate. The Raman spectrum confirms the formation of the 1T phase of TaS2. Modulation depths of 69.3 and 46.8% have been achieved at 0.1 THz and 0.9 THz frequency, respectively, under a low pumping power of 1 W/cm2. A constant higher modulation depth in the wide frequency range reveals the broadband response of the THz modulator. Under the same conditions, the modulation increased twice as compared to bare Si after annealing at 300 °C in the presence of air. Furthermore, numerical analysis based on the finite-difference time domain shows that a greater number of photogenerated charge carriers are present near the interface of Si and TaS2, which leads to enhancement in modulation. The utilization of 1T-TaS2 imparts potential to these TMDs in the wide THz frequency range and unfolds the possibilities for their use in THz imaging, wireless communication, and detection processes.

Nanometer-Thick Bismuth Nanocrystal Films for Sensoric Applications

The present article is concerned with investigations of the structural, surface morphological, and magnetotransport properties of DC magnetron-sputtered nanometer-thick Bi nanocrystal films on Si(111) substrates. Crystal structure and surface morphology were studied with X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, and atomic force microscopy. For the samples deposited at the melting point of Bi, 271 °C, equilibrium crystals formed and according to Wulff theorem acquire a specific shape determined by the surface tension. These crystals were investigated for different film thicknesses and deposition temperatures varying from 25 to 300 °C. Furthermore, magnetotransport characterization was carried out in steady and pulsed magnetic fields of up to 9 and 70 T, respectively. At low temperatures, clear weak antilocalization behavior is observed, attributed to 2D conduction channels. A nonlinear Hall resistance is also confirmed, ascribed to the coexistence of two types of carriers (p and n). This study contributes to the elucidation of the transport properties of the Bi thin films and opens new perspectives for their exploitation in modern applications such as sensorics.

Science and Technology of Integrated Super-High Dielectric Constant AlOx/TiOy Nanolaminates / Diamond for MOS Capacitors and MOSFETs

A super-high dielectric constant AlOx/TiOy nanolaminate film is grown on hydrogenated diamond (H-diamond) to enable superior metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs). In order to minimize or suppress leakage current, a nanometer thick AlOx film is inserted at the AlOx/TiOy nanolaminate/H-diamond interface. The maximum values for the capacitance density and dielectric constant related to the summation of individual AlOx and nanolaminate are 1.06 μF/cm2 and 68.7, respectively. Capacitance density and dielectric constant for the AlOx/TiOy nanolaminate are as high as 5.22 μF/cm2 and 308, respectively. Electrical properties of four H-diamond MOSFETs with gate lengths increasing from 2.4 μm to 10.1 μm were investigated. All of them showed p-type behavior and distinct pinch-off characteristics with drain current maxima of −47.4, −43.3, −26.6, and −24.6 mA/mm, respectively. On/off ratios and threshold voltages for the MOSFETs are higher than 104 and lower than 0.55 ± 0.10 V, respectively. The low threshold voltages indicate that the AlOx/TiOy nanolaminate gate-based MOSFETs can switch between ON and OFF stages at low gate voltages. Effective mobilities of the H-diamond channel layers for all the MOSFETs raised firstly and dropped subsequently with increasing voltages, which can be explained by the effect of mobility limiting factors.

Clusters of protein pores in phospholipid bilayer membranes can be identified and characterized by electrochemical impedance spectroscopy

Cluster formation is a widely-observed phenomenon, which results in unique sets of properties both in physical, chemical and biological domains of nature. Here, we present a qualitative description of clustering of membrane defects, specifically ion-conducting protein pores in tethered phospholipid bilayers, a versatile biomimetic model of biological membranes accomplished on chemically modified gold films. By invoking the Voronoi tessellation concept we demonstrate the possibility to distinguish between random and sparsely clustered patterns both in computer generated and real-world systems using one single parameter σ, the standard deviation of the normalized Voronoi sector areas distribution. For random systems, σ ≈ 0.54, for sparsely clustered patterns σ > 0.54. Because of a specific structure and dielectric properties of tethered bilayers, they can be characterized by an alternating current technique, electrochemical impedance spectroscopy (EIS). EIS measures macroscopic parameters of dielectric systems and it is not a structural technique per se. However, we found the EIS spectra-derived quantitative metric ζ to be a diagnostic parameter that allows assessment of the distribution type (homogeneous, random or clustered) of defects at nanometer level. One of the most interesting findings of the current study is the fact that the EIS derived ζ parameter is sensitive to an average size of the defects thus enabling a purely electrochemical methodology to access fine structural information such as size of incomplete protein pores in phospholipid bilayers. Overall, our results demonstrate a fundamental property of the macroscopic technique, electrochemical impedance spectroscopy, to probe structural arrangement of defects with sizes between 0.5 nm to 25.5 nm located in a thin, 2 nm thick phospholipid dielectric layer. Our findings can be utilized in designing precision electrochemical biosensors, and/or solving specific physical, biochemical or electrochemical problems related to other types of nanometer thick dielectric films on conducting surfaces.

Effects of alloying elements on surface oxides of hot–dip galvanized press hardened steel

The effects of steel alloying elements on the formation of the surface oxide layer of hot–dip galvanized press hardened steel after austenitization annealing were examined with various advanced microscopy and spectroscopy techniques. The main oxides on top of the original thin Al2O3 layer, originating from the primary galvanizing process, were identified as ZnO and (Mn,Zn)Mn2O4 spinel. For some of the investigated steel alloys, an additional non–uniform, several nanometer thick Cr enriched film was found at the Al2O3 layer. At a sufficiently high concentration, Cr can act as a substitute for Al during annealing, strengthening and regenerating the original Al2O3 layer with Cr2O3. Further analysis with secondary ion mass spectrometry allowed a reliable distinction between ZnO and Zn(OH)2.

Measurement of the transmission secondary electron yield of nanometer-thick films in a prototype Timed Photon Counter

We measure the transmission secondary electron yield of nanometer-thick Al2O3/TiN/Al2O3 films using a prototype version of a Timed Photon Counter (TiPC) . We discuss the method to measure the yield extensively. The yield is then measured as a function of landing energy between 1.2 and 1.8 keV and found to be in the range of 0.1 (1.2 keV) to 0.9 (1.8 keV). These results are in agreement to data obtained by a different, independent method. We therefore conclude that the prototype TiPC is able to characterise the thin films in terms of transmission secondary electron yield. Additionally, observed features which are unrelated to the yield determination are interpreted.

Structure-resistive property relationships in thin ferroelectric BaTiO_{3} films

A combined study of local structural, electric and ferroelectric properties of SrTiO_{3}/La_{0.7}Sr_{0.3}MnO_{3}/BaTiO_{3} heterostructures was performed by Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy in the temperature range 30–295 K. The direct correlation of film structure (epitaxial, nanocrystalline or polycrystalline) with local electric and ferroelectric properties was observed. For polycrystalline ferroelectric films the predominant polarization state is defined by the peculiarity of screening the built-in field by positively charged point defects. Based on Scanning Tunneling Spectroscopy results, it was found that a sequent voltage application provokes the modification of local resistive properties related to the redistribution of point defects in thin ferroelectric films. A qualitative analysis of acquired Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy images together with Scanning Tunneling Spectroscopy measurements enabled us to conclude that in the presence of structural defects the competing processes of electron injection, trap filling and the drift of positively charged point defects drives the change of resistive properties of thin films under applied electric field. In this paper, we propose a new approach based on Scanning Tunneling Microscopy/Spectroscopy under ultrahigh vacuum conditions to clarify the influence of point defects on local resistive properties of nanometer-thick ferroelectric films.

Detection of ferromagnetic resonance from 1\xa0nm-thick Co

To explore the further possibilities of nanometer-thick ferromagnetic films (ultrathin ferromagnetic films), we investigated the ferromagnetic resonance (FMR) of 1 nm-thick Co film. Whilst an FMR signal was not observed for the Co film grown on a SiO2 substrate, the insertion of a 3 nm-thick amorphous Ta buffer layer beneath the Co enabled the detection of a salient FMR signal, which was attributed to the smooth surface of the amorphous Ta. This result implies the excitation of FMR in an ultrathin ferromagnetic film, which can pave the way to controlling magnons in ultrathin ferromagnetic films.

Fast, wafer-scale growth of a nanometer-thick graphite film on Ni foil and its structural analysis

The growth of graphite on polycrystalline Ni by chemical vapor deposition (CVD) and the microstructural relation of the graphitic films and the metallic substrate continues to puzzle the scientific community. Here, we report the wafer-scale growth of a nanometer-thick graphite film (∼100 nm, NGF) on Ni foil via a fast-thermal CVD approach (5 min growth). Moreover, we shed light on how localized thickness variations of the NGF relate to the Ni surface topography and grain characteristics. While on a macro-scale (mm2), the NGF film looks uniform—with a few hundred highly ordered graphene layers (d0002 = 0.335 nm), when studied at the micro- and nano-scales, few-layer graphene sections can be identified. These are present at a density of 0.1%–3% areas in 100 µ m2, can be as thin as two layers, and follow an epitaxial relation with the {111} fcc-Ni planes. Throughout the 50 cm2 NGF, the sharp graphite/substrate interfaces are either composed of a couple of NiCx layers or a graphene layer. Moreover, the NGF was successfully transferred on SiO2/Si substrate by a wet chemical etching method. The as-produced NGFs could complement or offer an alternative to the mm-thick films produced from natural graphite flakes or polymer sheets.

Effect of transverse dissipative particle dynamics on dynamic properties of nanometer-thick liquid films on solid surfaces

ABSTRACT To ascertain the effect of transverse dissipative particle dynamics (DPD), which includes lateral dissipative forces in addition to the central ones in the standard DPD, on dynamics of systems involving interfaces, we compare dynamic properties derived from coarse-grained (CG) molecular dynamics simulations with the standard and transverse DPD for nanometer-thick liquid films on solid surfaces. The dynamic properties include relaxation times of the film thickness distribution and molecular rotational motion, and transport properties associated with molecular translational motion. Our results show that these dynamic properties are tuneable by changing friction coefficients in the transverse DPD, whereas this is not the case in the standard DPD. We also confirm that the transverse DPD applied to liquid–solid CG bead pairs can tune dynamic properties of nanometer-thick liquid films on solid surfaces, though it is less effective than when applied to liquid–liquid CG bead pairs. Moreover, we reveal that the dissipative forces are isotropic in transverse DPD, whereas the liquid–solid dissipative forces are highly anisotropic and nearly along the direction perpendicular to solid surfaces in standard DPD. This suggests that the transverse liquid–solid DPD might be crucial for modelling the in-plane energy dissipation at liquid–solid interfaces.

Nanometer-Thick Nickel Oxide Films Prepared from Alanine-Chelated Coordination Complexes for Electrochromic Smart Windows

The use of coordination complexes as metal precursors in electrodeposition of transition-metal oxides has been shown to be important for film formation at the nanoscale. Here, we present a systemat...

Quantitative Anisotropic Analysis of Molecular Orientation in Amorphous N2O at 6 K by Infrared Multiple-Angle Incidence Resolution Spectrometry.

The existence of molecular orientational order in nanometer-thick films of molecules has long been implied by surface potential measurements. However, direct quantitative determination of the molecular orientation is challenging, especially for metastable amorphous thin films at low temperatures. This study quantifies molecular orientation in amorphous N2O at 6 K using infrared multiple-angle incidence resolution spectrometry (IR-MAIRS). The intensity ratio of the weak antisymmetric stretching vibration band of the 14N15NO isotopomer between the in-plane and out-of-plane IR-MAIRS spectra provides an average molecular orientation angle of 65° from the surface normal. No discernible change is observed in the orientation angle when a different substrate material is used (Si and Ar) at 6 K or the Si substrate temperature is changed in the range of 6-14 K. This suggests that the transient mobility of N2O during physisorption is key in governing the molecular orientation in amorphous N2O.

Macro-, Micro- and Nano-Roughness of Carbon-Based Interface with the Living Cells: Towards a Versatile Bio-Sensing Platform.

Integration of living cells with nonbiological surfaces (substrates) of sensors, scaffolds, and implants implies severe restrictions on the interface quality and properties, which broadly cover all elements of the interaction between the living and artificial systems (materials, surface modifications, drug-eluting coatings, etc.). Substrate materials must support cellular viability, preserve sterility, and at the same time allow real-time analysis and control of cellular activity. We have compared new substrates based on graphene and pyrolytic carbon (PyC) for the cultivation of living cells. These are PyC films of nanometer thickness deposited on SiO2 and black silicon and graphene nanowall films composed of graphene flakes oriented perpendicular to the Si substrate. The structure, morphology, and interface properties of these substrates are analyzed in terms of their biocompatibility. The PyC demonstrates interface biocompatibility, promising for controlling cell proliferation and directional intercellular contact formation while as-grown graphene walls possess high hydrophobicity and poor biocompatibility. By performing experiments with C6 glioma cells we discovered that PyC is a cell-friendly coating that can be used without poly-l-lysine or other biopolymers for controlling cell adhesion. Thus, the opportunity to easily control the physical/chemical properties and nanotopography makes the PyC films a perfect candidate for the development of biosensors and 3D bioscaffolds.

Highly conductive nanometer-thick gold films grown on molybdenum disulfide surfaces for interconnect applications

Thin gold (Au) films (10 nm) are deposited on different substrates by using a e-beam deposition system. Compared with sapphire and SiO2 surfaces, longer migration length of the Au adatoms is observed on MoS2 surfaces, which helps in the formation of a single-crystal Au film on the MoS2 surface at 200 °C. The results have demonstrated that with the assistance of van der Waals epitaxy growth mode, single-crystal 3D metals can be grown on 2D material surfaces. With the improved crystalline quality and less significant Au grain coalescence on MoS2 surfaces, sheet resistance 2.9 Ω/sq is obtained for the thin 10 nm Au film at 100 °C, which is the lowest value reported in literature. The highly conductive thin metal film is advantageous for the application of backend interconnects for the electronic devices with reduced line widths.

Nanometer-Thick Films of Aligned ZnO Nanowires Sensitized with Au Nanoparticles for Few-ppb-Level Acetylene Detection

Wireless devices for environmental and health monitoring require the development of stable and highly sensitive thin metal oxide gas-sensing films. This paper reports one-step Langmuir–Blodgett assembly of ultrathin (20 nm) nanostructured films of aligned ZnO nanowires with 10 nm-wide v-grooves on the surface, followed by sensitization with Au nanoparticles (Au NPs) by sputtering and postannealing. The ridges of the nanopatterns are revealed as diffusion barriers capable of preventing Au NPs from sintering via particle migration and growth (PMC). The resulting ZnO Langmuir–Blodgett film decorated with high dispersion of small Au NPs functions as a highly responsive conductance switch, which demonstrates unprecedented sensitivity (37 ppm–1) and detection limit (3 ppb) toward acetylene (C2H2), a key marker gas for air pollution caused by anthropogenic emission. The fabrication method for the stabilized nanometric metal on the Langmuir–Blodgett film reported in this work not only provides a general strategy to improve the sensitivity of the thin sensing film but also can be applied to many metal nanodot-based applications, such as solar cells, plasmon-resonance waveguides, and biosensors.

Three-omega thermal-conductivity measurements with curved heater geometries

The three-omega method, a powerful technique to measure the thermal conductivity of nanometer-thick films and the interfaces between them, has historically employed straight conductive wires to act as both heaters and thermometers. When investigating stochastically prepared samples such as two-dimensional materials and nanomembranes, residue and excess material can make it difficult to fit the required millimeter-long straight wire on the sample surface. There are currently no available criteria for how diverting three-omega heater wires around obstacles affects the validity of the thermal measurement. In this Letter, we quantify the effect of the wire curvature by performing three-omega experiments with a wide range of frequencies using both curved and straight heater geometries on SiO2/Si samples. When the heating wire is curved, we find that the measured Si substrate thermal conductivity changes by only 0.2%. Similarly, we find that wire curvature has no significant effect on the determination of the thermal resistance of an ∼65 nm SiO2 layer, even for the sharpest corners considered here, for which the largest measured ratio of the thermal penetration depth of the applied thermal wave to radius of curvature of the heating wire is 4.3. This result provides useful design criteria for three-omega experiments by setting a lower bound for the maximum ratio of the thermal penetration depth to wire radius of curvature.

Nanometer-thick films of antimony oxide nanoparticles grafted on defective graphenes as heterogeneous base catalysts for coupling reactions

Films of few-layers defective N-doped or undoped graphene (10–15 nm) containing antimony oxide nanoparticles (15–30 nm) have been prepared on quartz by pyrolysis of alginate or chitosan adsorbing Sb(OAc)3. XPS shows that the prevalent Sb oxidation state is +III, while thermoprogrammed CO2 desorption shows that these films exhibit basic sites. These thin films have used as basic catalysts to promote the Michael addition of active methylene compounds and the Henry condensation. These results have been rationalized by DFT calculations that have shown that undercoordinated or two-fold coordinated oxygen atoms on SbOx clusters can act as basic sites, providing a wide range of basic strength.

A Fluoropolymer-Coated Nanometer-Thick Cu Mesh Film for a Robust and Hydrophobic Transparent Heater

To develop more effective optoelectronic devices with a transparent heater, it is necessary to investigate the droplet evaporation characteristics and wettability control on the heater surface. An optically transparent and hydrophobic amorphous fluoropolymer, Cytop, is spin-coated onto a nano-thick copper (Cu) micromesh-based transparent conductor to evaluate its performance for a high-durable transparent heater (a transmittance of 81.6% at 550 nm, a sheet resistance of 5 Ω sq–1). As the result, the thermal and chemical stabilities of the pure Cu micromesh-based transparent heater with the ∼80 nm thick Cytop layer improve without performance degradation. The evaporation time of water droplets on the surface of the hydrophobic transparent heater is noticeably delayed (about 60–130%) because the average evaporation flux of the droplet on the surface of the hydrophobic transparent heater is lower than those on the hydrophilic heater surfaces. In addition, unlike when the heater surface is hydrophilic, there is no coffee-ring effect when the heater surface is hydrophobic due to the recirculating Marangoni flow within the droplet. Further, the hydrophobic transparent heater surface exhibits excellent icephobic and antifrost properties. The results will be helpful for the further development of practical transparent heaters including self-cleaning smart windows, transparent actuators, transparent chemical and biological sensors, and transparent heating sources.