Surface Electrical Properties Research Articles

Temperature-dependent optical and surface electrical properties of Pb(Zr0.3Ti0.7)O3 /p-GaN film

Understanding the temperature-dependent optical and electrical properties of PZT, a multifunctional ferroelectric thin film with temperature sensitivity, is crucial for its applications. This study systematically investigated the microstructure, optical, and surface electrical features of Pb(Zr0.3Ti0.7)O3 thin film deposited on p-GaN substrate (PZT/p-GaN), with an emphasis on their response to temperature variations. Band gap energy (3.643 eV) and the three interband electronic transitions (3.528 eV, 4.662 eV, 6.582 eV) were extracted from optical measurements, which govern the photoelectron transmission behavior of the PZT/p-GaN. Additionally, we identify three phase-transition temperatures (450 K, 550 K, and 620 K) through temperature-dependent optical behavior analysis. With increasing temperature, the electronic transitions and bandgap show a redshift trend, and the bandgap change follows the O'Donnell equation with a significant electron-phonon coupling coefficient (S = 10.284), revealing a strong electron-phonon coupling effect in PZT/p-GaN. The temperature-dependent kelvin piezoelectric force microscopy (KPFM) shows that the surface potential and work function of PZT/p-GaN exhibit different linear variations over three temperature ranges divided by the phase transition temperature point as a demarcation. Furthermore, we observed that the optical and surface electrical properties exhibit anomalous trends at the phase transition point. These findings offer a theoretical reference for the application of PZT thin films in optoelectronics and electronic devices.

Comparative investigation of surface-electrical properties of functionalized graphene and MXene thin films for CO2 gas sensing

Comparative investigation of surface-electrical properties of functionalized graphene and MXene thin films for CO2 gas sensing

Effects of wet chemical etching on surface band bending and electrical properties of Sn-doped β-Ga2O3 (101) substrate

Effects of wet chemical etching on surface band bending and electrical properties of Sn-doped β-Ga2O3 (101) substrate

Analysis of the Characteristics of Mesh with Complex Knitting Patterns for Spaceborne Reflector Antennas

Metallic mesh with complex woven structures is commonly used as the surface in large aperture spaceborne reflector antennas. To study the electrical properties of mesh surfaces characterized by intricate patterns, this paper presents a method for building models to analyze complex meshes. High Frequency Structure Simulator and Computer Simulation Technology (CST) simulation of the reflection coefficients, with Astrakhan’s formulation as a reference, confirm that applying the wire-grid model in CST using the tetrahedral mesh type provides sufficiently accurate results. Furthermore, the rectangular periodic wire-grid model is simulated in CST, and its results are compared with those of Astrakhan’s formulation. Correlations between reflection characteristics and influence factors, including mesh opening size, wire diameter, nature of wire contact, and wave incident angle, are investigated. To exemplify the applicability of the wire-grid model to complex mesh with irregular pattern shapes, the reflection coefficients of a warp-knitted gold-coated molybdenum mesh woven by multi-wires are measured and compared to the simulated results from the wire-grid model. The results prove that the method proposed in this paper is suitable for modeling metal mesh with complex weave patterns.

Experimental investigations on metallization in the surface modified additively manufactured plastic substrates using DC sputtering

The application of copper surface coating to plastic structures offers numerous advantages, including high thermal and electrical conductivity, improved mechanical properties, good corrosion resistance, decorative applications, and enhancements in working temperatures. Besides these advantages, producing plastic structures with 3D printing and applying surface coating enables the final structures to become functional plastic structures adaptable to different fields. In this study, 3D plastic structures were produced using the fused deposition modeling method. Pristine, dichloromethane dipping, dichloromethane vapor, cold oxygen plasma, and mechanical abrasion surface treatments were applied to determine the optimal surface treatment between copper and the plastic substrate before copper coating. Subsequently, copper coating on plastic structures was completed using the DC sputtering technique. The surface topography, optical, electrical, and structural properties of the produced plastic structures were examined. According to X-ray diffraction analysis results, the (111), (200), (220), and (311) crystal planes confirm the presence of copper. The electrical conductivity values of the plastic structures reached 7.87 × 105 S/m. Contact angle measurement results indicate that the applied surface treatments increased the contact angles to 88.309°, leading the coated plastic structures to exhibit a more hydrophobic behavior.

Atomistic Insights into the Ionic Response and Mechanism of Antifouling Zwitterionic Polymer Brushes.

Zwitterionic polymer brushes are not a practical choice since their ionic response mechanisms are unclear, despite their great potential for surface antifouling modification. Therefore, atomic force microscopy and molecular dynamics simulations investigated the ionic response of the surface electrical properties, hydration properties, and protein adhesion of three types of zwitterionic brushes. The surface of PMPC (poly(2-methacryloyloxyethyl phosphorylcholine)) and PSBMA (poly(sulfobetaine methacrylate)) zwitterionic polymer brushes in salt solution exhibits a significant accumulation of cations, which results in a positive shift in the surface potential. In contrast, the surface of PSBMA polymer brushes demonstrates no notable change in potential. Furthermore, divalent Ca2+ enhances protein adhesion to polymer brushes by Ca2+ bridges. Conversely, monovalent Na+ diminishes the number of salt bridges between PSBMA and PCBMA (poly(carboxybetaine methacrylate)) zwitterionic polymer brushes and proteins via a competitive adsorption mechanism, thereby reducing protein adhesion. A summary of polymer brush material selection and design concepts in a salt solution environment is provided based on the salt response law of protein adhesion resistance of various zwitterionic materials. This work closes a research gap on the response mechanism of zwitterionic polymer brushes' antifouling performance in a salt solution environment, significantly advancing the practical use of these brushes.

The effects of sub-monolayer laser etching on the chemical and electrical properties of the (100) diamond surface

Tailoring the surface chemistry of diamond is critical to a range of applications from quantum science to electronics. It has been recently shown that dosing the diamond surface with pulsed UV light at fluences below the ablation threshold provides a practical method for precision etching of the surface. Here, we track the evolution of the surface chemistry and its electrical properties as a function of dose using x-ray surface analysis, Hall and resistance measurements. It is found that the surface properties evolve rapidly, even for doses that correspond to removal of less than 5% of the top carbon monolayer and fluences less than 1 J/cm2. As well altering XPS-measured surface populations, sub-monolayer etch doses lower the valence band by up to 0.2 eV, and produce a permanent increase in the conductivity of the hydrogen terminated surface by up to 7 times. Similar enhancements in conductivity are obtained for doses that remove up to 1600 ML. The results provide guidance for manipulating diamond surface chemistry by UV laser etching and introduce a promising method for enhancing the performance of diamond devices such as field-effect transistors.

Surface electrical properties of UHPFRC with graphene

The role of graphene inclusion in ultra-high-performance concrete (UHPC) and in ultra-high-performance fiber reinforced concrete (UHPFRC) and the role of steel fibres (used in UHPFRC) have been studied. Surface electrical properties were shown as useful means to deeply investigate the properties of UHPFRC including graphene. Due to the inclusion of fibers a marked increasing of surface conductivity was shown in UHPFRC respect to UHPC. Surface conductivity has been estimated ∼20 % lower than bulk conductivity in mature UHPFRC. A method for assessing the dispersion grade of graphene in concrete, based on surface electrical measurements and numerical modelling, is presented. In case of mature UHPC, increasing surface conductivity with increasing the graphene content was observed. In case of UHPFRC, graphene within the investigated range of contents (≤1 % bwoc) was found not to greatly affect surface conductivity. With including graphene, a reduction of microstructural disorder was estimated, whereas the inclusion of fibers was estimated to increase the microstructural disorder.

Fabrication and characterization of bioresorbable, electroactive and highly regular nanomodulated cell interfaces

Biomaterial-based implantable scaffolds capable of promoting physical and functional reconnection of injured spinal cord and nerves represent the latest frontier in neural tissue engineering. Here, we report the fabrication and characterization of self-standing, biocompatible and bioresorbable substrates endowed with both controlled nanotopography and electroactivity, intended for the design of transient implantable scaffolds for neural tissue engineering. In particular, we obtain conductive and nano-modulated poly(D,L-lactic acid) (PLA) and poly(lactic-co-glycolic acid) free-standing films by simply iterating a replica moulding process and coating the polymer with a thin layer of poly(3,4-ethylendioxythiophene) polystyrene sulfonate. The capability of the substrates to retain both surface patterning and electrical properties when exposed to a liquid environment has been evaluated by atomic force microscopy, electrochemical impedance spectroscopy and thermal characterizations. In particular, we show that PLA-based films maintain their surface nano-modulation for up to three weeks of exposure to a liquid environment, a time sufficient for promoting axonal anisotropic sprouting and growth during neuronal cell differentiation. In conclusion, the developed substrates represent a novel and easily-tunable platform to design bioresorbable implantable devices featuring both topographic and electrical cues.

Improved Corona Resistance of Epoxy Resin for Gas‐Insulated Equipment in Nitrogen Gas by Direct Fluorination

Direct fluorination has demonstrated efficacy in modulating the surface electrical properties of epoxy insulators and preventing surface charge accumulation. Comparative corona treatments were conducted on both surface fluorinated (modified) epoxy samples and untreated (virgin/original) samples using a modified electrode setup in a nitrogen (N2) atmosphere. The purpose was to evaluate the resistance of the modified epoxy surface layer against corona discharge. The results of ATR‐IR analysis and SEM surface and cross‐sectional imaging indicate that corona treatment did not alter the chemical composition of the fluorinated sample or the thickness of the fluorinated layer. Based on assessments of potential decay and water contact angle measurements, the surface conductivity of the modified sample was decreased by corona treatment. The wettability of fluorinated sample remained unchanged after corona treatment, and no soluble degradation products were produced. Conversely, the surface conductivity of the virgin sample exhibited an increase following corona treatment, accompanied by the formation of soluble degradation products. These results confirm that direct fluorination improves the epoxy insulator's discharge resistance in N2 and provides technical support for enhancing the electrical performance of epoxy insulators in gas insulated equipment. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

The Impact of Thermal Treatment on the Structural, Optical and Electrochemical Characteristics of Tin Sulfide Films

The development of eco-friendly, cost-effective, and naturally abundant electrode materials for supercapacitors is gaining critical importance in current energy storage research. This study focuses on the synthesis of tin sulfide (SnSx) films via the eco-friendly successive ionic layer adsorption and reaction (SILAR) method, employing varying quantities of L-ascorbic acid (0.8 and 1.0 g) as a reducing agent. Tin sulfide films were deposited on fluorine-doped tin oxide (FTO) glass substrates and subsequently annealed in an inert atmosphere at temperatures ranging from 200 to 400 °C, resulting in thin films of varying thicknesses (100–420 nm). The structural and compositional characteristics of the films were thoroughly analyzed using Raman spectroscopy to confirm the purity and spectroscopic signatures of the sulfides. Further characterization was performed to assess the films’ morphology (scanning electron microscopy, SEM), phase composition (X-ray diffraction, XRD), surface chemical states (X-ray photoelectron spectroscopy, XPS), optical properties (UV–Vis spectroscopy), and electrical properties (Hall measurements). The gathered data were then used to evaluate the potential of tin sulfide films as electrode materials in supercapacitors, highlighting their suitability for sustainable energy storage applications.

Influence of Si flow rate on the performance of MOCVD-deposited Si-doped Ga2O3 films and the applications in ultraviolet photodetectors

In this work, the Metal-organic Chemical Vapor Deposition (MOCVD) technology was used to successfully grow Si-doped β-Ga2O3 films on C-plane sapphire substrates. The effects of Si flow rate on the surface morphology, crystal composition, electrical and optical properties of the films were characterized and analyzed. The experimental results show that the full width at half maximum (FWHM) and root mean square (RMS) of the films are improved with the decrease of Si flow rate. More importantly, only the sample with the lowest Si flow rate showed conductive ability, and its carrier concentration and mobility were 4.20 cm2/V·s and 3.33 × 1016 cm−3, respectively. In addition, we also made photodetectors corresponding to the thin films. The test results showed that the external quantum efficiency (EQE) and responsiveness (R) of the detectors improved with the decrease of Si flow rate.

Exploring the diagnostic and prognostic significance of circulating tumor cells in stage II-IV colorectal cancer using a nano-based detection method.

Colorectal cancer (CRC) is a leading cause of cancer mortality globally, underscoring the urgency for a noninvasive and effective biomarker to enhance patient prognosis. Circulating tumor cells (CTCs), a potential marker for real-time tumor monitoring, are limited in clinical utility due to the low sensitivity of existing detection methods. Previously, we introduced a novel nano-based CTCs detection method that relies on the electrical properties of cell surfaces, thus eliminating the need for specific molecular biomarkers. In this study, we used this technique to evaluate the diagnostic and prognostic value of CTCs in stage II-IV CRC. A total of 194 participants were included, consisting of 136 CRC patients and 58 healthy individuals. The peripheral blood of the participants was collected, and CTC enumeration was performed utilizing the nano-based detection method that we newly developed. The receiver operating characteristic (ROC) curve and multivariate Cox proportional-hazards analysis were used to assess the effectiveness of CTCs for diagnosing CRC and predicting patient prognosis. The nano-based method demonstrated an ability to differentiate CRC patients from healthy individuals with a sensitivity of 84.6% and a specificity of 94.8%. Furthermore, baseline CTC levels were predictive of progression-free survival (PFS) in CRC patients, with lower levels associated with longer PFS compared to higher levels (4.5 vs 8.0 months at 15 CTCs/mL, p = 0.016; 4.4 vs 8.0 months at 20 CTCs/mL, p = 0.028). We also explored the dynamic changes in the number of CTCs after 1 to 5 cycles of chemotherapy. Patients with increasing CTC levels typically experienced disease progression (PD), while those with decreasing levels often achieved a partial response (PR) or maintained stable disease (SD). These findings suggest that the dynamic fluctuations in CTC counts are closely tied to the clinical course of the disease. Our study indicates the potential of nano-based CTCs detection in diagnosing and predicting outcomes for patients with stage II-IV CRC.

The graphene-on-diamond structure with Ni-catalyzed under high temperature

Graphene-on-diamond (GOD) composite structure has been attracting considerable attention due to its unique features for all carbon sp3-sp2 electronic applications. Whereby the electrical properties of diamond surface can be purposely tailored and significantly altered through transformed graphene layers. In this work, GOD composite structures were prepared by nickel (Ni)-catalyzed high-temperature rapid annealing, and were analyzed by Raman, Hall effect measurement and Transmission electron microscopy (TEM). The results show that the difference in surface conductivity of GOD composite structure is mainly related to the number of transformed graphene layers, which is mainly affected by annealing temperature, annealing time and the thickness of nickel film. Hall measurement and TEM results demonstrate that when the transformed graphene grows graphite with a lot of Ni atoms embedded into diamond, the surface carriers of GOD composite structure are electrons. On the contrary, the surface carriers are holes while the transformed graphene contains about 3 or 5 layers. These findings may provide a route for GOD structure to become a strong candidate for next generation complementary diamond electronic devices.

Conductive Atomic Force Microscopy-Ultralow-Current Measurement Systems for Nanoscale Imaging of a Surface's Electrical Properties.

One of the most advanced and versatile nanoscale diagnostic tools is atomic force microscopy. By enabling advanced imaging techniques, it allows us to determine various assets of a surface, including morphological, electrical, mechanical, magnetic, and thermal properties. Measuring local current flow is one of the very important methods of evaluation for, for instance, photovoltaic materials or semiconductor structures and other nanodevices. Due to contact areas, the current densities can easily reach above 1 kA/m2; therefore, special detection/measurement setups are required. They meet the required measurement range, sensitivity, noise level, and bandwidth at the measurement scale. Also, they prevent the sample from becoming damaged and prevent unwanted tip-sample issues. In this paper, we present three different nanoscale current measurement solutions, supported with test results, proving their performance.

(Nanocarbons Division SES Young Investigator Award) Surface Chemistry of Nanocarbon Field-Effect Transistors: From Fundamental Physics to Biosensing

Conductive carbon nanomaterials are particularly well-suited to form field-effect biosensors (bioFETs) because they are stable in aqueous media and can be chemically coupled to biomolecules. Their reduced dimensionality, either 2D (graphene) or 1D (carbon nanotubes), is key to their sensitivity: because they are entirely surfaces, these materials present remarkable electrical transport properties that are easily altered by microscopic surface interactions. Biochemical processes occurring in close proximity to their surface can therefore be transduced as changes in their electrical conductance. Selectivity for specific biochemical events, however, must be engineered via functionalization of the nanomaterial, i.e. by modifying its surface with bioactive or biorecognition elements (e.g., antibodies, aptamers, enzymes, polymer layer). Our group recently analyzed a corpus of studies presenting graphene field-effect transistors (GFETs) as bioanalytical sensors for the quantitative detection of biomarkers in the form of nucleic acids, proteins, ions or small molecules [1]. We found a striking variance in the reported sensing performance between these studies, that could not be explained by controlling for differences in graphene material, analyte type and for evolution over the years. Our hypothesis is that the lack of microscopic control on the biointerface is likely the most important source of variation in such bioFETs, highlighting the critical need to improve our knowledge and control of the functionalization of nanocarbon materials.In this talk, I will discuss our work investigating the interplay between surface chemistry and electrical properties in nanocarbon field-effect transistors. In the first part, I will focus on covalent additions using aryldiazonium salts. I will review what we have learned on the fundamental physics of this chemistry from experiments and simulations on carbon nanotube devices [2-5], in particular on its disruptive character because of the formation of sp 3 defects in the carbon lattice. More recently, we investigated this same chemistry on GFET devices and observed a heterogeneous response in their electrical conductance. Aiming to solve this, we developed a novel approach to precisely trigger and stop the reaction in situ using the applied gate voltage [6], enabling unprecedented control on the functionalization rate and yield on GFETs. In the second part, I will describe our ongoing efforts to characterize and control the biofunctionalization of GFETs through non-covalent pyrene derivatives, using a combination of electrical measurements, microscopy and spectroscopy techniques, and theoretical simulations. Finally, I will discuss our ongoing translational projects in oncology, on adapting nanocarbon-based bioFETs in cancer diagnostics applications.[1] Béraud et al, Analyst 146 (2), 403-428 (2021)[2] Bouilly et al, Applied Physics Letters 101 (5), 053116 (2012)[3] Bouilly et al, ACS nano 9 (3), 2626-2634 (2015)[4] Bouilly et al, Nano Letters 16 (7), 4679-4685 (2016)[5] Côté et al, Physical Chemistry Chemical Physics 24 (7), 4174-4186 (2021)[6] Bazan et al, Nano Letters 22 (7), 2635-2642 (2022)

Multilayered carbon nanotube/adhesive films for human body signal detection sensors

This paper presents the development of multilayered carbon nanotube (CNT)/adhesive (MLCA) films for human body signal detection sensors using a spray-coating method. The chemical composition, adhesion properties, and electrical conductivity of the films were investigated using various adhesives, with the acrylate-based adhesive (ABA) exhibiting superior performance. The surface roughness, thickness, and electrical properties of the films were characterized, and the tunability was demonstrated by adjusting the number of coating layers. Tribological tests were performed to assess the wear resistance and friction behavior of the films. The adhesion stabilities and conformabilities of the films on various substrates were investigated. The films were combined with polydimethylsiloxane (PDMS) and surfactants to create biocompatible and durable sensors. The PDMS-surfactant composite was characterized, and the MLCA film/PDMS-surfactant-based sensor exhibited excellent stability under deformation and biocompatibility. The impedance behavior, temperature, humidity, and strain-sensing capabilities of the sensors were evaluated. The capability of the sensor to detect vital signs was validated by accurately capturing the electrocardiogram (ECG) waveforms. This study provides valuable insights into the design and fabrication of CNT-based conductive films for human body signal-detection sensors, offering a promising approach for the development of flexible and wearable electronic devices.

Engineering polyvinylidene fluoride membranes using charge tunable dendrimer polyamidoamine to enable microporous membranes for protein separation

Dendrimers are structurally precise molecules, whose peripheral layer provides ample sites for anchoring functional groups of different charges and charge densities. This characteristic renders them capability of providing numerous target functional groups on the surface of filtration membranes when modified by dendrimers. Consequently, there has been a growing interest in dendrimer-modified membranes. In this study, we investigated the engineering of polyvinylidene fluoride (PVDF) membranes using polyamidoamine (PAMAM) dendrimers anchoring different functional groups to tune the charges and charge density of membranes to enable microporous membranes for separation of charged macromolecules. PVDF membranes were functionalized by grafting PAMAM-G3.0 (generation 3) dendrimers with amine groups onto their surfaces and internal surfaces of membrane pores. Subsequently, the peripheral functional groups of dendrimers on the membranes were replaced by carboxyl groups, quaternary ammonium groups, or sulfonic acid groups for charges of different nature and densities. Surface chemical properties, electrical properties, and morphology were characterized using various techniques, including attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and surface zeta potential measurement. The filtration performance of both PVDF and modified membranes was evaluated using polyethylene oxide (PEO), polyacrylic acid (PAA), and whey protein filtration. Among the modified membranes, the G3PA membranes, with carboxyl functional groups, exhibited the most effective whey protein separation performance. The whey protein rejection and permeate flux of the G3PA membranes were 79 % and 30.3 LMH, respectively. As a comparison, the whey protein rejection and permeate flux of the pristine PVDF membranes were 58.9 % and 15.3 LMH, respectively.

Polyvinylidene fluoride-based loose nanofiltration membrane with graphene oxide intercalation for efficient small organic molecules desalination

Graphene oxide (GO) loose nanofiltration (LNF) membranes show immense potential for the desalination of small organic molecules. However, it remains a significant challenge to compare the performance of GO LNF membranes prepared via different doping methods and study the mechanisms that influence the separation abilities of the membranes. In this study, LNF membranes prepared via three GO doping methods were examined for flux, retention rate, and the separation performance of small organic molecules and inorganic salts. The membranes coated with dopamine (DA) and GO, and subsequently formed polydopamine (PDA)–GO intercalation, showed the best results. A novel LNF membrane material (PVDF/PDA–GO–TFN; PVDF: polyvinylidene fluoride, TFN: thin-film nanocomposite) was developed to enhance the separation performance of small organic molecules and inorganic salts while improving membrane surface stabilization and anti-fouling performance and reducing the “trade-off” effect. The optimal conditions for PDA–GO intercalation and the best combination for interfacial polymerization (IP) were also determined. The intercalation structure was identified via scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and Fourier-transform infrared spectroscopy. Furthermore, the separation mechanism of the membrane was determined to be based on the Donnan and steric hindrance effects, according to the correlation analysis of the membrane surface electrical properties (zeta potential), hydrophilicity (contact angle), and pore properties. The PVDF/PDA–GO–TFN LNF membrane displayed a maximum water permeance of 10.45 L/(m2·h·bar), with an 88 % rejection of PEG2000 and a 6.83 % rejection of Na2SO4. Moreover, the membrane exhibited excellent anti-fouling performance and long-term stability, with a mean flux recovery rate of 99.06 % after four cycles of filtration and backwashing tests using small organic molecules and inorganic salt solutions. The inorganic salt retention rate was analyzed using SPSS software, and the results indicated excellent membrane stability. In summary, the preparation of PVDF/PDA–GO–TFN membrane materials provides a novel approach for fabricating LNF membranes that can effectively separate small organic molecules and inorganic salts.

Comparative study of sodium and potassium compounds as promoters for growth of monolayer MoS2 with high crystal quality on SiO2/Si substrate

Monolayer MoS2 is promising candidate for fabrication of optoelectronic devices due to its direct bandgap nature and high carrier mobility. Alkali metal compounds have been demonstrated to be helpful promoters for the growth of large single crystal monolayer MoS2 on SiO2/Si substrate. However, the catalytic mechanism of alkali metal compounds is still under debate. Herein, we compared the surface morphology, optical properties, and electrical properties of monolayer MoS2 flakes grown on SiO2/Si substrate assisted by promoters containing potassium or sodium cations and halogen (chlorine) or non-halogen (hydroxide) anions, i.e. NaCl, NaOH, KCl and KOH. Based on the analysis of existing growth mechanism, we proposed that the alkali metal cation, plays a dominant role in promoting the lateral growth of monolayer MoS2 and obtaining high crystal quality. Furthermore, potassium has a greater promoting effect than sodium. By optimizing growth conditions, monolayer triangular MoS2 flakes with large lateral size over 160 μm were grown assisted by KCl promoter. Raman and PL spectra verified excellent crystal quality of the flakes, with typical electron mobilities of 2.98 and 20 cm2 V−1 s−1 for the back-gated filed effect transistors fabricated on as-grown and fresh SiO2/Si substrates, respectively.