Raman Signal Intensity Research Articles

Probing the Tribochemical Impact on Wear Rate Dynamics of Hydrogenated Amorphous Carbon via Raman-Based Profilometry.

Solid-liquid lubricating systems have received significant attention as a promising way for energy saving and emission control. For deeply understanding their tribological behaviors, it is necessary to study interaction mechanisms between solid and liquid lubricants from the tribochemical viewpoint, as tribofilms formed by tribochemical products on contact surfaces critically affect the whole tribological process. Continually or periodically monitoring tribofilm formation and evolution can contribute significantly to clarifying its dominating role in tribological behavior under boundary lubrication. However, detecting tribofilms in situ remains a big challenge for conventional surface analytical approaches, mainly due to their limitations in accessing tribofilms or low signal intensities of thin tribofilms. In this study, highly sensitive Raman-based profilometry with in situ potential has been developed for detecting molybdenum dialkyldithiocarbamate (MoDTC)-derived tribofilms and exploring their effect on a-C:H wear over time. The optical properties of tribochemical products formed on the coating surface in different wear stages could result in extra attenuation of Raman signal intensities in the form of measurement deviations in wear depth. By monitoring the deviations, key information of tribofilm compositions was obtained and a two-stage wear progression mechanism was proposed for the first time to clarify the detrimental effect of MoDTC-derived tribofilms on a-C:H wear by combining detailed structure and composition analyses.

Flexible 2D nanocellulose-based SERS substrate for pesticide residue detection

Nanocellulose holds considerable promise as an effective surface-enhanced Raman scattering (SERS) substrate for sensitive detection of trace targets. Flexible and high-sensitivity two-dimensional (2D) SERS substrates based on nanocrystalline cellulose (CNC) film were successfully developed via self assembly of two plasma nanoparticles: gold nanoflowers (AuNFs) and silver-coated gold nanocubes (Au@AgNCs). The loading process allows the precise control of nanoparticle distribution density and uniformity on CNC film, which are closely related to the plasma coupling effect between particles. The obtained CNC/Au@AgNC flexible two-dimensional substrate could sensitively detect pesticide residues on apple surface, and the detection limits (LOD) of dimethoate and acetamiprid were 4.1 and 10.7 μg/L, respectively. In addition, Raman signal intensity showed a good linear relationship with pesticide concentration in the range of 10–100 μg/L, which provided great potential for high sensitivity and field detection of dangerous targets.

Hazardous Gas Detection by Cavity-Enhanced Raman Spectroscopy for Environmental Safety Monitoring.

We demonstrate the practicability of cavity-enhanced Raman spectroscopy (CERS) with a folded multipass cavity as a unique tool for the detection of hazardous gases in the atmosphere. A four-mirror Z-sharped multipass cavity results in a greatly extended laser-gas interaction length to improve the Raman signal intensity of gases. For Raman intensity maximization, the optimal number of intracavity beams of a single reflection cycle is calculated and then the cavity parameters are designed. A total of 360 intracavity beams are realized, which are circulated four times in the cavity based on the polarization. ppb-Level Raman gas sensing at atmospheric pressure for several typical explosive gases and toxic gases in ambient air, including hydrogen (H2), methane (CH4), carbon monoxide (CO), hydrogen sulfide (H2S), and chlorine (Cl2), is achieved at 300 s exposure time. Our CERS apparatus, which can detect multiple gases simultaneously with ultrahigh sensitivity and high selectivity, is powerful for detecting hazardous gases in the atmosphere, and it has excellent potential for environmental safety monitoring.

Cancer Cell-Specific Enhanced Raman Imaging and Photothermal Therapeutic Effect Based on Reversibly pH-Responsive Gold Nanoparticles.

Stimuli-responsive nanoparticles are favorable for improving the selective delivery and rational vocation that easily avoids the undesirable barriers or side effects, leading to a further improved therapeutic efficiency. Furthermore, multifunctional nanomaterials have been extensively developed as attractive candidates for theranostic reagents for cancer treatment. In this article, we developed reversibly pH-responsive gold nanoparticles (AuNPs) with an enhanced Raman scattering signal as well as an efficient photothermal effect and demonstrated their applications as a theranostic reagent for cancer treatment. Surfaces of these AuNPs were modified with mixed layers of Cy3-modified single-stranded DNA (ssDNA-Cy3) for Raman probing and a negative charge supply and cytochrome C (Cyt C) for pH-responsive charge inversion. This combination of pH-responsive ligands and Raman probes played an important role in inducing the assembly or disassembly of AuNPs corresponding to the neighboring pH, accompanied by an additional highly distinguished Raman signal intensity. An operative reversible response of the AuNPs to pH is endowed with the characteristic behavior of AuNPs with the cancerous cell's acidic microenvironment of low pH. The responsive aggregation of AuNPs in a lower pH medium provides highly amplified signals attributed to well-formed hot spots between the particle surfaces that deliver better Raman scattering signals. The acidic pH-responsive aggregation of the particles also provided efficient photothermal treatments using a long-wavelength laser light with the benefit of deeper penetration for cancer cells. In vitro experiments employing cancer cells and control normal cells well-demonstrated the specificity of the particles to cancer cells in terms of highly enhanced Raman imaging and therapeutic efficiency.

Dimensional reduction based on peak fitting of Raman micro spectroscopy data improves detection of prostate cancer in tissue specimens.

.Significance: Prostate cancer is the most common cancer among men. An accurate diagnosis of its severity at detection plays a major role in improving their survival. Recently, machine learning models using biomarkers identified from Raman micro-spectroscopy discriminated intraductal carcinoma of the prostate (IDC-P) from cancer tissue with a detection accuracy and differentiated high-grade prostatic intraepithelial neoplasia (HGPIN) from IDC-P with a accuracy.Aim: To improve the classification performance of machine learning models identifying different types of prostate cancer tissue using a new dimensional reduction technique.Approach: A radial basis function (RBF) kernel support vector machine (SVM) model was trained on Raman spectra of prostate tissue from a 272-patient cohort (Centre hospitalier de l’Université de Montréal, CHUM) and tested on two independent cohorts of 76 patients [University Health Network (UHN)] and 135 patients (Centre hospitalier universitaire de Québec-Université Laval, CHUQc-UL). Two types of engineered features were used. Individual intensity features, i.e., Raman signal intensity measured at particular wavelengths and novel Raman spectra fitted peak features consisting of peak heights and widths.Results: Combining engineered features improved classification performance for the three aforementioned classification tasks. The improvements for IDC-P/cancer classification for the UHN and CHUQc-UL testing sets in accuracy, sensitivity, specificity, and area under the curve (AUC) are (numbers in parenthesis are associated with the CHUQc-UL testing set): (), (), (6%), () with respect to the current best models. Discrimination between HGPIN and IDC-P was also improved in both testing cohorts: (), (), (), (). While no global improvements were obtained for the normal versus cancer classification task [ (), (), (), ()], the AUC was improved in both testing sets.Conclusions: Combining individual intensity features and novel Raman fitted peak features, improved the classification performance on two independent and multicenter testing sets in comparison to using only individual intensity features.

Discriminative and quantitative analysis of norepinephrine and epinephrine by surface-enhanced Raman spectroscopy with gold nanoparticle suspensions.

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique capable of increasing the Raman signal of an analyte using specific nanostructures. The close contact between those nanostructures, usually a suspension of nanoparticles, and the molecule of interest produces an important exaltation of the intensity of the Raman signal. Even if the exaltation leads to an improvement of Raman spectroscopy sensitivity, the complexity of the SERS signal and the numbers of parameters to be controlled allow the use of SERS for detection rather than quantification. The aim of this study was to develop a robust discriminative and quantitative analysis in accordance with pharmaceutical standards. In this present work, we develop a discriminative and quantitative analysis based on the previous optimized parameters obtained by the design of experiments fixed for norepinephrine (NOR) and extended to epinephrine (EPI) which are two neurotransmitters with very similar structures. Studying the short evolution of the Raman signal intensity over time coupled with chemometric tools allowedthe identification ofoutliers and their removal from the data set. The discriminant analysis showed an excellent separation of EPI and NOR. The comparative analysis of the data showed the superiority of the multivariate analysis after logarithmic transformation. The quantitative analysis allowed the development of robust quantification models from several gold nanoparticle batches with limits of quantification of 32µg/mL for NOR and below 20µg/mL for EPI even though no Raman signal is observable for such concentrations. This study improves SERS analysis over ultrasensitive detection for discrimination and quantification using a handheld Raman spectrometer.

SERS substrates based on self-organized dimple nanostructures on polyethylene naphthalate films produced via oxygen ion beam sputtering

Surface-enhanced Raman spectroscopy (SERS) utilizes metal nanostructures to enhance the intensity of Raman signals. Although many methods have been developed for fabricating SERS nanostructures, most involve multiple steps. Herein, we employed oxygen ion beam sputtering (IBS), a one-step technique suitable for processing flexible substrates in roll-to-roll processes for mass production, Specifically, one-step oxygen IBS was used to fabricate self-organized dimple nanostructures, whose area and roughness could be controlled using the ion irradiation energy density, on the surfaces of polyethylene naphthalate films. Gold nano-tips for SERS were subsequently obtained by evaporating gold onto the dimple nanostructures. Finite-difference time-domain (FDTD) simulations revealed that nano-tip structures with spacings of less than 10 nm increased the localized E-field enhancement, which improved the SERS signal. Fabrication at a low energy density (5.8 J/cm2) produced more nano-tips with spacings of less than 10 nm, corresponding to a density of 61.4 nano-tips/μm2. SERS analysis conducted with methylene blue at 638 nm and 785 nm demonstrated that the Raman signal intensity was stronger for SERS substrates fabricated with low energy density (5.8 J/cm2) than for substrates fabricated with high energy density (17.3 J/cm2), because of the high density of nano-tips on the former substrate.

Fiber-enhanced Raman spectroscopy for highly sensitive H2 and SO2 sensing with a hollow-core anti-resonant fiber.

An innovative fiber-enhanced Raman gas sensing system with a hollow-core anti-resonant fiber is introduced. Two iris diaphragms are implemented for spatial filtering, and a reflecting mirror is attached to one fiber end that provides a highly improved Raman signal enhancement over 2.9 times than the typical bare fiber system. The analytical performance for multigas compositions is thoroughly demonstrated by recording the Raman spectra of carbon dioxide (CO2), oxygen (O2), nitrogen (N2), hydrogen (H2), and sulfur dioxide (SO2) with limits of detection down to low-ppm levels as well as a long-term instability < 1.05%. The excellent linear relationship between Raman signal intensity (peak height) and gas concentrations indicates a promising potential for accurate quantification.

A Novel Multiplex Mycotoxin Surface-Enhanced Raman Spectroscopy Immunoassay Using Functional Gold Nanotags on a Silica Photonic Crystal Microsphere Biochip.

A novel multiplex mycotoxin surface-enhanced Raman spectroscopy (SERS) immunoassay was established for the first time on different artificial antigen-modified silica photonic crystal microspheres (SPCMs), which can be integrated into a biochip array to achieve multiplex detection using corresponding antibody-functionalized gold nanoparticles (AuNPs) as the SERS nanotag. The unique optical structure of SPCMs is helpful to find the detection spots easily, accommodate a large amount of probe molecules, and enhance the Raman signal intensity. Such enhancement was confirmed by the simulation result, showing the electric field enhancing effect in SPCMs with AuNPs being 7 times. A competitive SERS immunoassay was established using antigen-modified SPCMs and mycotoxins to compete for binding antibody-functionalized SERS nanotags, displaying broad linear detection ranges of 0.001-0.1 ng/mL for aflatoxin B1 (AFB1), 0.01-10 ng/mL for ochratoxin A (OTA), and 0.001-0.1 ng/mL for zearalenone (ZEN) and low detection limits of 0.82 pg/mL for AFB1, 1.43 pg/mL for OTA, and 1.00 pg/mL for ZEN. In the spiked cereal samples, recovery rates of the method were measured in the range of 70.35-118.04% for the three mycotoxins, which was in agreement with that of the traditional enzyme-linked immunosorbent assay method. The SERS immunoassay for mycotoxin detection also showed high specificity and good repeatability and reproducibility. The new microsphere-based SERS immunoassay biochip only requires a one-step reaction and overcomes the disadvantages of fluorescence and chemiluminescence background signals. The work paves the way for further developing SERS-based microsphere suspension arrays for new targets.

Influence of laser shape on thermal increase during micro‐Raman spectroscopy analyses

AbstractMicro‐Raman spectrometers are used in various domains, from archaeology to space exploration. These systems use microscope objectives to focus the laser onto the sample to be analysed. Although this method drastically increases the spatial resolution down to a few hundreds of nanometres, it is also commonly admitted that this focused laser beam may heat and alter the studied material. The setting of the laser power is thus a universal problem for any micro‐Raman spectroscopy user who has to find a compromise between the Raman signal intensity and the risk of thermal alteration. This parameter is not easy to set based on bibliographic references because, depending on the architecture of the system used, on the laser wavelength and on the sample, damage may occur at very different laser power levels. Here, we describe the different parameters that influence thermal increase induced by the laser and propose easy to make experiments to measure them. A series of experiments is then carried out to study the thermal alteration of various materials when exposed to a laser beam focused with the different microscope objectives of a micro‐Raman system. It is shown that, except for very thin samples or powdered materials, the relevant parameters to consider are the laser power at the sample surface and the duration of exposure to the laser beam, whatever the objective used. We also demonstrate that the risk of sample alteration is almost non‐existent for thick transparent materials. Interestingly, it is shown that, for semi‐transparent materials, alteration may occur at lower laser power for low magnification objectives than for high magnification objectives. These results are explained by the high aperture angle of microscope objectives which induces high dispersion of the laser within the sample.

Charge-Transfer Process in Surface-Enhanced Raman Scattering Based on Energy Level Locations of Rare-Earth Nd3+-Doped TiO2 Nanoparticles.

Surface-enhanced Raman scattering (SERS) for semiconductor nanomaterial systems is limited due to weak Raman signal intensity and unclear charge-transfer (CT) processes for chemical enhancement. Here, rare-earth element neodymium-doped titanium dioxide (Nd-TiO2) nanoparticles (NPs) were synthesized by the sol–gel method. The characterizations show that the doping of Nd ions causes TiO2 NPs to show an increase in the concentration of defects and change in the energy level structure. The CT process between Nd-TiO2 NPs substrate and probe molecule 4-Mercaptopyridine (4-Mpy) was innovatively analyzed using the relative energy level location relationship of the Dorenbos model. The SERS signal intensity exhibits an exponential enhancement with increasing Nd doping concentration and reaches its optimum at 2%, which is attributed to two factors: (1) The increase in the defect concentration is beneficial to the CT process between the TiO2 and the probe molecule; (2) the introduction of 4f electron orbital energy levels of rare-earth ions created unique CT process between Nd3+ and 4-Mpy. Moreover, the Nd-TiO2 NPs substrate shows excellent SERS performance in Raman signal reproducibility (RSD = 5.31%), the limit of detection (LOD = 10−6 M), and enhancement factor (EF = 3.79 × 104). Our work not only improves the SERS performance of semiconductor substrates but also provides a novel approach to the development of selective detection of probe molecules.

Reusable dual-enhancement SERS sensor based on graphene and hybrid nanostructures for ultrasensitive lead (Ⅱ) detection

Lead (Pb2+) is the most common toxic heavy metal ion in the natural environment and causes enormous harm to public health for its non-metabolic accumulation. Therefore, it is essential to develop sensitive and robust system for detecting lead ions. Here, we provide a dual-enhancement (DE) surface-enhanced Raman scattering (SERS) strategy, that integrate the chemical enhancement of monolayer graphene and the electromagnetic enhancement of hybrid metal nanostructure, for sensitive quantitative detection of Pb2+ through enzyme-linked reaction principle. The enhancement factor (EF) of 3.85 × 108 of the DE SERS substrate was achieved. Pb2+ ions in water can be detected quantitatively through recording the Raman signal intensity of the Cy3. The results show that the developed SERS sensor owns a wide dynamic response range between 10 pM and 100 nM with a detection limit of 4.31 pM for Pb2+ detection. Furthermore, this high-quality SERS sensor possesses high Pb2+ specificity and can be recycled at least three times. In addition, the sensing mechanism of the SERS sensor is discussed and validated by the experiments and finite-difference time-domain (FDTD) simulations. The proposed dual-enhancement, reusable SERS sensor provides an effective approach for heavy ion detection and gene analysis in the field of environmental and life science and technology.

A Bioorthogonal Probe for Multiscale Imaging by 19F-MRI and Raman Microscopy: From Whole Body to Single Cells.

Molecular imaging techniques are essential tools for better investigating biological processes and detecting disease biomarkers with improvement of both diagnosis and therapy monitoring. Often, a single imaging technique is not sufficient to obtain comprehensive information at different levels. Multimodal diagnostic probes are key tools to enable imaging across multiple scales. The direct registration of in vivo imaging markers with ex vivo imaging at the cellular level with a single probe is still challenging. Fluorinated (19F) probes have been increasingly showing promising potentialities for in vivo cell tracking by 19F-MRI. Here we present the unique features of a bioorthogonal 19F-probe that enables direct signal correlation of MRI with Raman imaging. In particular, we reveal the ability of PERFECTA, a superfluorinated molecule, to exhibit a remarkable intense Raman signal distinct from cell and tissue fingerprints. Therefore, PERFECTA combines in a single molecule excellent characteristics for both macroscopic in vivo19F-MRI, across the whole body, and microscopic imaging at tissue and cellular levels by Raman imaging.

Silicon Microchannel-Driven Raman Scattering Enhancement to Improve Gold Nanorod Functions as a SERS Substrate toward Single-Molecule Detection.

The investigation of enhanced Raman signal effects and the preparation of high-quality, reliable surface-enhanced Raman scattering (SERS) substrates is still a hot topic in the SERS field. Herein, we report an effect based on the shape-induced enhanced Raman scattering (SIERS) to improve the action of gold nanorods (AuNRs) as a SERS substrate. Scattered electric field simulations reveal that bare V-shaped Si substrates exhibit spatially distributed interference patterns from the incident radiation used in the Raman experiment, resulting in constructive interference for an enhanced Raman signal. Experimental data show a 4.29 increase in Raman signal intensity for bare V-shaped Si microchannels when compared with flat Si substrates. The combination of V-shaped microchannels and uniform aggregates of AuNRs is the key feature to achieve detections in ultra-low concentrations, enabling reproducible SERS substrates having high performance and sensitivity. Besides SIERS effects, the geometric design of V-shaped microchannels also enables a “trap” to the molecule confinement and builds up an excellent electromagnetic field distribution by AuNR aggregates. The statistical projection of SERS spectra combined with the SIERS effect displayed a silhouette coefficient of 0.83, indicating attomolar (10–18 mol L–1) detection with the V-shaped Si microchannel.

Molybdenum Oxide/Tungsten Oxide Nano-heterojunction with Improved Surface-Enhanced Raman Scattering Performance.

By virtue of their high uniformity and stability, metal oxide-based surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attention for molecular trace detection. However, strategies for further enhancing the SERS sensitivity are still desired. Herein, MoOx/WOx nano-heterojunctions are constructed by mixing MoOx and WOx together (MoOx/WOx hybrid) with diverse weight ratios. Using a 532 nm laser as the excitation source and R6G as the Raman reporter, it is shown that the Raman signal intensity (for the peak @ 1360 cm-1) obtained on the optimal MoOx/WOx hybrid (MoOx/WOx = 1:1/3) is twice that observed on a pure MoOx or WOx substrate. Moreover, a limit of detection of 10-8 M and an enhancement factor of 108 are achieved. In the SERS enhancement mechanism investigation, it is revealed that MoOx and WOx form a staggered band structure. During the SERS measurement, electron-hole pairs are generated in the nano-heterojunction using the incident laser. They are then separated by the built-in potential with the electrons moving toward WOx. The accumulated electrons on WOx are further transferred to the R6G molecules through the coupling of orbitals. Consequently, the molecular polarizability is amplified, and SERS performance is enhanced. The abovementioned explanation is supported by the evidence that the contribution of the chemical enhancement mechanism in the optimal MoOx/WOx hybrid substrate is about 2.5 times or 5.9 times that in the pure WOx or MoOx substrate.

Harnessing abruptly auto-defocusing beam to enhance the Raman signal in aqueous humor: A simulation analysis

Raman aqueous humor detection provides a non-invasive, molecular-level approach for ingredient analysis within aqueous humor. However, current Raman aqueous humor applications are facing low signal-to-noise levels due to the trade-off between laser power and laser safety. In order to increase Raman signal while guaranteeing laser safety, in the research, we propose to use the abruptly auto-defocusing (AADF) beam as the illumination source for Raman aqueous humor spectroscopy. The ray-tracing sketch together with the propagation simulation of AADF shows its evolution within the aqueous humor. The intensity distributions are analyzed with and without the impact of corneal refraction. Results show that the efficiency of AADF is higher than the conventional focused Gaussian beam (FGB) method with center blocked. The peak value of the Raman signal intensity acquired using the AADF method is about 6 times larger than that of an FGB.

Flexible hydrophobic filter paper-based SERS substrate using silver nanocubes for sensitive and rapid detection of adenine

Flexible paper-based surface enhanced Raman spectroscopy (SERS) substrate has attracted great attention in the sensitive detection of analytes in recent years. However, due to its inherent hydrophilic surface, the nanoparticles will not be concentrated very well on the surface of the paper. This reduces the reproducibility and sensitivity of the paper-based substrate. Herein, we demonstrate flexible hydrophobic paper-based substrate using silver nanocubes loaded on polymethylhydrosiloxane modified filter paper (AgNCs@SiO2/PMHS modified filter paper) to realize high sensitivity and reproducibility. The hydrophobic filter paper was obtained by dip-soaking the filter paper into PMHS solution, resulting in increased contact angle to ~112.5°. The AgNCs@SiO2/PMHS modified paper substrate shows high analytical enhancement factor (AEF) of 6.55 × 106, which is 2.2-fold enhancement compared to AgNCs@SiO2/normal paper substrate, and a low detection limit of 0.1 nM rhodamine 6G and 0.89 nM adenine. The high SERS enhancement factor and sensitivity is attributed to the high hotspots density created in the hydrophobic surface as a result of the excellent uniformity and superior sample collection efficiency compared to the normal filter paper-based substrate. Besides, the substrate shows excellent spot-to-spot and substrate-to-substrate reproducibility, with relative standard deviation (RSD) of 6.05% and 9.22%, respectively. Moreover, the AgNCs@SiO2/PMHS modified filter paper substrate achieved good stability; the Raman signal intensity only drops by 25.88% after 3 months of exposure to air. Therefore, this flexible hydrophobic AgNCs@SiO2/PMHS modified filter paper substrate with low cost fabrication and label-free SERS detection is a promising SERS platform for analytical and biological sensing applications.

Raman Microspectroscopic Analysis of Selenium Bioaccumulation by Green Alga Chlorella vulgaris.

Selenium (Se) is an element with many commercial applications as well as an essential micronutrient. Dietary Se has antioxidant properties and it is known to play a role in cancer prevention. However, the general population often suffers from Se deficiency. Green algae, such as Chlorella vulgaris, cultivated in Se-enriched environment may be used as a food supplement to provide adequate levels of Se. We used Raman microspectroscopy (RS) for fast, reliable, and non-destructive measurement of Se concentration in living algal cells. We employed inductively coupled plasma-mass spectrometry as a reference method to RS and we found a substantial correlation between the Raman signal intensity at 252 cm−1 and total Se concentration in the studied cells. We used RS to assess the uptake of Se by living and inactivated algae and demonstrated the necessity of active cellular transport for Se accumulation. Additionally, we observed the intracellular Se being transformed into an insoluble elemental form, which we further supported by the energy-dispersive X-ray spectroscopy imaging.

New Insight into the Characterization of Graphene Oxide and Reduced Graphene Oxide Monolayer Flakes on Si-Based Substrates by Optical Microscopy and Raman Spectroscopy

Graphene oxide (GO) and reduced GO (RGO) have attracted interest as economical alternatives for pristine graphene with controllable properties spanning their applications in many fields. Abundant lattice defects present in GO and RGO have made their physical and chemical properties largely diverse from that of pristine graphene. This makes their identification and characterization ambiguous as the rules applied for pristine graphene may not be practical for GO and RGO. Here, we report a new insight into the characterization of GO and thermally reduced GO monolayer flakes deposited on different commonly used Si-based substrates by Raman spectroscopy along with optical visualization to address this uncertainty. It was found that the dielectric substrates with a lower reflectance in the visible range give a better optical contrast under normal white light illumination and intense Raman signals owing to the multiple reflection and interference at the interfaces of the multilayer system. Accordingly, a Si substrate with a 90 nm SiO2 layer gives the highest IG′/IG ratio along with a better optical visibility for monolayer RGO flakes.

Interfacial stress characterization of GaN epitaxial layer with sapphire substrate by confocal Raman spectroscopy

As an important wide-bandgap semiconductor, gallium nitride (GaN) has attracted considerable attention. This paper describes the use of confocal Raman spectroscopy to characterize undoped GaN, n-type GaN, and p-type GaN through depth profiling using 405-, 532-, and 638-nm wavelength lasers. The Raman signal intensity of the sapphire substrate at different focal depths is studied to analyze the depth resolution. Based on the shift of the E2H mode of the GaN epitaxial layer, the interfacial stress for different types of GaN is characterized and calculated. The results show that the maximum interfacial stress appears approximately at the junction of the GaN and the sapphire substrate. Local interfacial stress analysis between the GaN epitaxial layer and the substrate will be very helpful in furthering the applications of GaN devices.