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Enhancement of graphene-related and substrate-related Raman modes through dielectric layer deposition
Applied Physics Letters
Abstract
In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method’s efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface.
... Intermediary effects methods help to get below this level 9 . In the case of the PRISM method, it is possible to reach an accuracy of single nanometers 10,11 for thin dielectric layers or even a single-atom layer for graphene coating 12 . ...
... For this PRISM method mode, we have prepared microstructures in a homoepitaxial layer of SiC in the form of an isosceles cross (dimensions of 20 µm × 90 µm ) etched with reactive ions. The nitrogen-doped 5.34-µm-thick homoepitaxial layer was grown on a 15 mm × 15 mm sample cut from a 3-in conducting, n-type (4×10 18 cm -3 ), 364-µm-thick 4H-SiC substrate (SiCrystal GmbH) with a 4 • off-cut from the basal vector [0001] towards the [11][12][13][14][15][16][17][18][19][20] direction. Silane and propane were used as precursors (C : Si = 1.8), and hydrogen as carrier gas 27 . ...
We demonstrate a genuine method for three-dimensional pictorial reconstructions of two-dimensional, three-dimensional, and hybrid specimens based on confocal Raman data collected in a back-scattering geometry of a 532-nm setup. The protocol, or the titular PRISM (Phase-Resolved Imaging Spectroscopic Method), allows for sub-diffractive and material-resolved imaging of the object’s constituent material phases. The spacial component comes through either the signal distal attenuation ratio (direct mode) or subtle light-matter interactions, including interference enhancement and light absorption (indirect mode). The phase component is brought about by scrutinizing only selected Raman-active modes. We illustrate the PRISM approach in common real-life examples, including photolithographically structured amorphous Al2O3, reactive-ion-etched homoepitaxial SiC, and Chemical Vapor Deposition graphene transferred from copper foil onto a Si substrate and AlGaN microcolumns. The method is implementable in widespread Raman apparatus and offers a leap in the quality of materials imaging. The lateral resolution of PRISM is stage-limited by step motors to 100 nm. At the same time, the vertical accuracy is estimated at a nanometer scale due to the sensitivity of one of the applied phenomena (interference enhancement) to the physical property of the material (layer thickness).
... It offers uneven growth conditions favoring additional graphene inclusions at SiC vicinal surfaces [7,8]. A detailed topographic analysis reveals further subtle structural inhomogeneities within the actual (0001) terraces [11,18]. All the above makes each QFS graphene on SiC(0001) sample structurally as unique as a fingerprint. ...
... Thin lamellas for the TEM observations were prepared by the Focused Ion Beam (FIB) technique using the FEI Helios 600 NanoLab Dual Beam Microscope equipped with the Omniprobe lift-out system. Since the TEM technique requires the deposition of a thin Pt layer, a process potentially hazardous to graphene, the sample surface was intentionally passivated with an 85-nm-thick [18] amorphous aluminum oxide layer synthesized from trimethylaluminum and deionized water in the process of atomic layer deposition. The sole purpose of the dielectric film was to secure the structural composition of graphene and provide a favorable visual contrast in the HR-TEM image [39]. ...
In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical
Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6H–SiC(0001),
characterized in that its room-temperature direct-current Hall-effect-derived hole mobility 𝜇p = 5019 cm2/Vs,
and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active
longitudinal optical A1 mode at 964 cm−1, equals N = 1.05. The distribution of the ellipsometric angle 𝛹
measured at an angle of incidence of 50◦ and 𝜆 = 490 nm points out to N = 0.97. The close-to-unity value of
N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further
confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which
include the Si–H peak at 2131 cm−1, the histograms of 𝛹 and 𝛥, and the Raman G and 2D band positions,
widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials.
... The test elements were manufactured using graphene-on-SiC technology [32] under the geometry and type available with the GET ® platform [44]. Each of the structures was a 1.4 mm × 1.4 mm fourterminal van der Pauw device [34] featuring an oxygen-plasma-etched, equal-arm, cross-shaped 100-μm × 300-μm QFS graphene mesa [45], electron-beam-deposited Ti/Au (10 nm/110 nm) current feed and voltage readout contacts, and a 100-nm-thick, atomic-layer-deposited, amorphous, non-stoichiometric, oxygen-deficient [46] Al 2 O 3 encapsulation [47,48]. The graphene was transfer-free, in-situ fully [49] hydrogen-intercalated [50,51] at 1273 K (therefore quasi-free-standing and p-type), and epitaxial Chemical Vapor Deposition [30,31]. ...
This article reveals a unique self-healing ability of the amorphous-aluminum-oxide-passivated p-type hydrogenintercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadiumcompensated nominally on-axis 6H-SiC(0001) system, exposed for 166 h to a destructive flux of 3.3 ×
E11 cm−2s−1 of mostly fast-neutrons (1–2 MeV), resulting in an accumulated fluence of 2.0 × E17 cm−2. Postirradiation room-temperature Hall effect characterization proves that the a-Al2O3/QFS-graphene/6H-SiC(0001) is n-type, which implies the loss of the quasi-free-standing character of graphene and likely damage to the SiC(0001)-saturating hydrogen layer. Micro-Raman spectroscopy suggests an average defect density in graphene of 𝑛𝐷 = 3.1 × 1010 cm−2 with an 𝐿𝐷 = 32-nm inter-defect distance. Yet, a thermal treatment up to 623 K eliminates defect-related Raman peaks and restores the original p-type conductance. At the same time, 623 K is not enough to recover the initial transport properties in a sample irradiated for 245 h with a total fluence of 2.0 × E18 cm−2. A Density Functional Theory model explains the self-healing phenomenon and restoration of the quasi-free-standing properties through thermally-activated lateral diffusion of the remaining population of hydrogen atoms and re-decoupling of the graphene sheet from the SiC(0001) surface. The thermal
regime of 623 K fits perfectly into the operational limits of the a-Al2O3/QFS-graphene/6H-SiC(0001) system, defined as 300 K to 770 K. The finding constitutes a milestone for two-dimensional, graphene-based diagnostic and control systems designed for operation in extreme environments
... The sample was processed into a batch of 96 van der Pauw structures [18]. Each structure was a 1.4-mm × 1.4-mm four-terminal device [12] featuring an oxygen-plasma-etched, equal-arm, cross-shaped [21] 100-µm × 300-µm graphene mesa, Ti/Au (10 nm/110 nm) current feed and voltage readout contacts, and a 100-nm-thick, atomic-layer-deposited, amorphous, non-stoichiometric [22] Al 2 O 3 passivation [23] synthesized from trimethylaluminum and deionized water at 770 K in the Picosun R-200 Advanced reactor. The choice of this specific geometry, rather than of an optimized Hall bar, was justified by the authors' will to elucidate the transport properties of the 5-keV H + -modified a-Al 2 O 3 /QFS-graphene/4H-SiC(0001) platform. ...
In this letter, we demonstrate a Hall effect sensor in the technology of amorphous-Al2O3-passivated transfer-free
p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity on-axis 4H-SiC(0001), pre-epitaxially modified with 5-
keV hydrogen (H+) ions. The sensor operates between 305 K and 770 K, with a current-mode sensitivity of ∼75 V/AT and thermal stability below 0.15 %/K (⩽ 0.03 %/K in a narrower range between 305 K and 700 K). It is a promising two-dimensional platform for high-temperature magnetic diagnostics and plasma control systems for modern tokamak fusion reactors.
... The pre-epitaxial ion modification was implemented in the technology of 1.4-mm × 1.4-mm Hall effect sensors, manufactured in the number of 96 out of each SI SiC substrate in the 20-mm × 20-mm standard, as previously introduced in Ref. [63]. Through a series of optical lithography-based steps involving metal electron-beam deposition, oxygen plasma etching and atomic layer deposition, the substrates were turned into van der Pauw structures, each featuring a cross-shaped [46] 100-μm × 300-μm QFS graphene mesa, Ti/Au (10 nm/90 nm) ohmic contacts, and a 100-nm-thick a-Al 2 O 3 passivation synthesized from trimethylaluminum (TMA) and deionized water at 770 K in a Picosun R200 Advanced ALD reactor [64]. Individual sensors were mounted onto and bonded to in-house-made 6.6-mm × 6.6-mm sapphire holders equipped with four Ti/Au (10 nm/190 nm) corner contacts enabling electrical characterization at = 1 mA in a 0.556-T direct-current Ecopia AHT55T5 automated Hall effect measurement system between 300 K and 770 K. Schematic of the Hall effect sensor is depicted in Fig. 3, while its real-life implementation evoked in the results and discussion section. ...
High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.
https://www.sciencedirect.com/science/article/pii/S2667056923000585
... The data suggest that analysis of the ratios between the Raman band intensities of the substrate and the PtSe 2 adlayer is the most reliable approach to determining the thickness of a small number of layers (up to 10 ML) of PtSe 2 covering the Al 2 O 3 substrate. A similar approach was used to investigate the defects in a graphene layer formed on an SiC substrate [44]. ...
The results of temperature-dependent Raman spectroscopy studies of thin layers of PtSe2 (1–10 monolayers) deposited on an Al2O3 substrate are disscused in this paper. A redshift of the vibrational Raman modes (Eg and A1g) is observed when the thickness of PtSe2 and temperature increase. The results allow for determining the thickness of the PtSe2 layer deposited on the Al2O3 substrate by analysing the Raman shift of the PtSe2 modes (Eg and A1g) and the screening effects on the surface vibration mode (A1g). The other original result is the determination of the stresses and doping effects in PtSe2 for the considered range of layer thicknesses by analyses of the correlative plot. Finally, the thermal dependences of the Raman spectra are discussed regarding Raman shifts and intensity. The atomic force microscopy measurements show the presence of residual contamination with surface densities varying between samples.
... First, we introduce an artificially-fabricated sample, or the rosette, containing a pre-defined pattern of a single, a double, and a triple graphene transfer from copper foil onto an oxidized silicon substrate. The rosette provides a graphene-free reference area for the normalization of the intensity of the Si-related mode at 520 cm −1 [25] (I Si ) and three areas of a monotonically increasing number of the graphene layers -all being the result of three subsequent overlapping transfers. We use the rosette structure to prove that the method is scalable up to at least five graphene layers. ...
In this report, we demonstrate an applied protocol for layer-resolved Raman imaging and analysis of undesirable ad-layers found in Chemical Vapor Deposition graphene grown on copper foil and transferred onto an oxidized silicon substrate. The method assumes that the intensity of the silicon-related Raman-active mode at 520 cm⁻¹ is attenuated by 2.3 % each time the light passes through a single graphene layer. Upon normalization with respect to a reference graphene-free area, the 520 cm⁻¹ mode relative intensity r-ISi measured in a back-scatter mode follows a univalent function of the number of the graphene layers N. Since N is treated as a continuous argument, it can be ascribed a fractional value and considered statistically. Importantly, the r-ISi offers higher layer differentiation capability and unambiguity than non-functional indicators, including the 2D band width or the 2D-to-G band intensity ratio, thus providing unequivocal evaluation.
... QFS graphene [26][27][28][29] necessary for the graphene-based HTHS was grown on semiinsulating high-purity on-axis 4H-SiC(0001) (Cree Inc.) in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition method [30], with thermally decomposed propane as a carbon source and in situ hydrogen atom intercalation [31]. The substrate was processed into a 1.6 mm × 1.6 mm Hall effect structure featuring a cross-shaped [32] 100 µm × 300 µm graphene mesa and four Ti/Au (10 nm / 60 nm) ohmic contacts, all passivated with a 100 nm-thick aluminum oxide (Al 2 O 3 ) layer synthesized from trimethylaluminum (TMA) and deionized water at 670 K in a Picosun R200 Advanced Atomic Layer Deposition (ALD) reactor [33,34]. Detailed information on the fabrication processes is included in Refs. ...
The ability to precisely measure magnetic fields under extreme operating conditions is becoming increasingly important as a result of the advent of modern diagnostics for future magnetic-confinement fusion devices. These conditions are recognized as strong neutron radiation and high temperatures (up to 350 °C). We report on the first experimental comparison of the impact of neutron radiation on graphene and indium antimonide thin films. For this purpose, a 2D-material-based structure was fabricated in the form of hydrogen-intercalated quasi-free-standing graphene on semi-insulating high-purity on-axis 4H-SiC(0001), passivated with an Al2O3 layer. InSb-based thin films, donor doped to varying degrees, were deposited on a monocrystalline gallium arsenide or a polycrystalline ceramic substrate. The thin films were covered with a SiO2 insulating layer. All samples were exposed to a fast-neutron fluence of ≈7×1017 cm⁻². The results have shown that the graphene sheet is only moderately affected by neutron radiation compared to the InSb-based structures. The low structural damage allowed the graphene/SiC system to retain its electrical properties and excellent sensitivity to magnetic fields. However, InSb-based structures proved to have significantly more post-irradiation self-healing capabilities when subject to proper temperature treatment. This property has been tested depending on the doping level and type of the substrate.
... The 2D mode at 2715 cm −1 corresponds to 622 nm and the G mode at 1596 cm −1 corresponds to 581 nm. For a given Al 2 O 3 thickness these lines experience different signal interference within the 100-nm-thick Al 2 O 3 passivation, and so does the I 2D /I G intensity ratio [43]. ...
In this report, we introduce a novel method based on low-frequency noise analysis for the assessment of quality and pattern of inhomogeneity in intentionally-aged Hall effect sensors featuring hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene mesa on semi-insulating high-purity on-axis 4H-SiC(0001), all passivated with a 100-nm-thick atomic-layer-deposited Al2O3 layer. Inferring from the comparison of the measured noise and one calculated for a homogeneous sensor, we hypothesize about possible unintentional contamination of the sensors’ active regions. Following in-depth structural characterization based on Nomarski interference contrast optical imaging, confocal micro-Raman spectroscopy, high-resolution Transmission Electron Microscopy and Secondary Ion Mass Spectrometry, we find out that the graphene’s quasi-free-standing character and p-type conductance make the Al2O3/graphene interface exceptionally vulnerable to uncontrolled contamination and its unrestrained lateral migration throughout the entire graphene mesa, eventually leading to the blistering of Al2O3. Thus, we prove the method’s suitability for the detection of these contaminants’ presence and location, and infer on its applicability to the investigation of any contamination-induced inhomogeneity in two-dimensional systems. https://authors.elsevier.com/a/1eyP64xMlkIhhc
... The substrate was processed into a batch of 25 1.6 mm × 1.6 mm Hall effect sensors [13,41] featuring a cross-shaped [39] 100 μm × 300 μm graphene mesa and four Ti/Au (10 nm/60 nm) ohmic contacts, all passivated with a 100-nm-thick aluminum oxide (Al 2 O 3 ) layer synthesized from trimethylaluminum (TMA) and deionized water at 670 K in a Picosun R200 Advanced ALD reactor [49]. The type of dielectric layer was selected for its noninvasive character, lack of energetic or reactive species, sub-nanometer precision, and layer uniformity. ...
In this paper, we report on the first experimental study on the impact of neutron radiation on quasi-free-standing (QFS) graphene. For this purpose, we have fabricated hydrogen-intercalated QFS graphene on semiinsulating high-purity 4H-SiC(0001), passivated it with an Al2O3 layer,and exposed it to a fast-neutron fluence of ≈6.6×1017 cm⁻². The results have shown that the graphene sheet is only moderately affected by the neutron radiation with the estimated defect density of ≈4×1010 cm⁻². The low structural damage allowed the Al2O3/graphene/SiC system to maintain its electrical properties and an excellent sensitivity to magnetic fields characteristic of QFS graphene. Consequently, our findings suggest that the system may be a promising platform for magnetic diagnostics in magnetic-confinement fusion reactors. However, the scope of its use should be a subject of further study. In this context, we have explored possible modes of damage and have concluded that the main factor that affects the electrical parameters of the structure is the impact of neutrons on the layer of hydrogen atoms saturating the SiC(0001) surface. We have shown, employing density functional theory (DFT) computations, that damage to the intercalating layer could lower hole concentration in graphene via reduced charge polarization and local coupling on the interface.
The relatively weak Raman enhanced factors of semiconductor-based substrate limit its further application in surface-enhanced Raman scattering (SERS). Here, a kind of two-dimensional (2D) semimetal material, molybdenum carbide (Mo2C) film, is prepared via a chemical vapor deposition (CVD) method, and the origin of SERS is investigated for the first time. The detection limits of the prepared Mo2C films for crystal violet (CV) and rhodamine 6G (R6G) molecules are low at 10-6 M and 10-8 M, respectively. Our detailed theoretical analysis, based on density functional theory and the finite element method, demonstrates that the enhancement of the 2D Mo2C film is indeed CM in nature rather than the EM effects. Besides, the basic doping strategies are proposed to further optimize the SERS sensitivity of Mo2C for Fermi level regulation. We believe this work will provide a helpful guide for developing a highly sensitive semimetal SERS substrate.
Enhancement of Raman scattering due to optical interference may act as a source of error in the issues necessitating a determination of Raman intensity. Its dependence on thin film thickness is the conventional way how to examine the effects of optical interference in Raman signal. To provide a new platform for evaluation of signal coming from substrate in the presence of a transparent capping, we investigate its relation to reflectance (R) instead of the capping thickness. We derived a theoretical model, which was experimentally tested on simple structures consisting of SiO2 deposited on mono‐Si substrates. In agreement between the model and the experiment, interference enhancement is proportional to the product of (1 − R) terms taken at excitation and scattered light wavenumbers. We experimented with two different Raman bands in Si on two different Raman systems. The model was valid regardless of excitation, Raman band, and grating. Constructed for normal incidence, it was in agreement with experiment using objectives with numerical apertures up to 0.25 (0.32). The model was valid also in ultraviolet region, where imaginary part in refractive index of Si considerably rises. Enhancement factor (EF) in Si substrate covered by SiO2 depends on reflectance (R) of the entire structure. We proposed a model of EF proportional to the product of (1 − R) terms taken at the wavenumbers of excitation line and studied Raman band. Constant of proportionality can be calculated from refractive indices.
In this report, we demonstrate a novel high-temperature Hall effect sensor that is based on quasi-free-standing monolayer graphene epitaxially grown on high-purity semiinsulating (SI) on-axis 4H-SiC(0001) substrate in a chemical vapor deposition process. To ensure statistical perspective, characteristics of 23 elements are determined as a function of temperature ranging from 300 to 770 K. Passivated with a 100-nm-thick atomic-layer-deposited aluminum oxide, the sensor offers current-mode sensitivity of 80 V/AT with thermal stability of -0.02%/K within the range between 300 and 573 K, and -0.06%/K between 573 and 770 K. The sensor's room-temperature output voltage is monitored in the magnetic field from -300 to +300 mT and its offset voltage at 0 T is assessed. Its high-temperature electrical properties are explained through a double-carrier transport involving spontaneous-polarization-induced holes in the graphene layer and thermally activated electrons emitted from a deep acceptor level related to silicon vacancy VSi^1-/2- occupying the k site of the 4H-SiC lattice. The sensor is compared with a previously reported one on vanadium-compensated SI on-axis 6H-SiC(0001). The new sensor's applicability to magnetic field detection at high temperatures is verified.
Atomic layer deposition (ALD) is the method of choice to obtain uniform insulating films on graphene for device applications. Owing to the lack of out‐of‐plane bonds in the sp² lattice of graphene, nucleation of ALD layers is typically promoted by functionalization treatments or predeposition of a seed layer, which, in turn, can adversely affect graphene electrical properties. Hence, ALD of dielectrics on graphene without prefunctionalization and seed layers would be highly desirable. In this work, uniform Al2O3 films are obtained by seed‐layer‐free thermal ALD at 250 °C on highly homogeneous monolayer (1L) epitaxial graphene (EG) (>98% 1L coverage) grown on on‐axis 4H‐SiC(0001). The enhanced nucleation behavior on 1L graphene is not related to the SiC substrate, but it is peculiar of the EG/SiC interface. Ab initio calculations show an enhanced adsorption energy for water molecules on highly n‐type doped 1L graphene, indicating the high doping of EG induced by the underlying buffer layer as the origin of the excellent Al2O3 nucleation. Nanoscale current mapping by conductive atomic force microscopy shows excellent insulating properties of the Al2O3 thin films on 1L EG, with a breakdown field > 8 MV cm⁻¹. These results will have important impact in graphene device technology.
We report Raman study of ultra thin Ge films using interference enhanced Raman scattering which uses a trilayer structure of Al, CeO2 and crystalline Ge films. The use of CeO2 allows the growth of crystalline Ge films at relatively low substrate temperatures (300°C). With a decrease of Ge film thickness, the Raman line exhibits an increased red shift of the peak position and line broadening. The latter can be quantitatively explained on the basis of phonon confinement in the growth direction. Raman spectra of the 2 nm and 4 nm thick Ge films show shoulder at ∼280 cm-1 which could be attributed to surface phonons. The changes in the Raman shift as a function of thickness showed that the films were compressively strained up to a thickness of ∼7 nm beyond which the strain is released.
There is little doubt that nanoparticles offer real and new opportunities in many fields, such as biomedicine and materials science. Such particles are small enough to enter almost all areas of the body, including cells and organelles, potentially leading to new approaches in nanomedicine. Sensors for small molecules of biochemical interest are of critical importance. This review is an attempt to trace the use of nanomaterials in biochemical sensor design. The possibility of using nanoparticles functionalized with antibodies as markers for proteins will be elucidated. Moreover, capabilities and applications for nanoparticles based on gold, silver, magnetic, and semiconductor materials (quantum dots), used in optical (absorbance, luminescence, surface enhanced Raman spectroscopy, surface plasmon resonance), electrochemical, and mass-sensitive sensors will be highlighted. The unique ability of nanosensors to improve the analysis of biochemical fluids is discussed either through considering the use of nanoparticles for in vitro molecular diagnosis, or in the biological/biochemical analysis for in vivo interaction with the human body.
Surface enhanced Raman spectroscopy (SERS) is a powerful vibrational
spectroscopy technique that allows for highly sensitive structural detection
of low concentration analytes through the amplification of electromagnetic
fields generated by the excitation of localized surface plasmons. SERS has
progressed from studies of model systems on roughened electrodes to
highly sophisticated studies, such as single molecule spectroscopy. We
summarize the current state of knowledge concerning the mechanism of
SERS and new substrate materials. We highlight recent applications of
SERS including sensing, spectroelectrochemistry, single molecule SERS,
and real-world applications. We also discuss contributions to the field
from the Van Duyne group. This review concludes with a discussion of
future directions for this field including biological probing with UV-SERS,
tip-enhanced Raman spectroscopy, and ultrafast SERS.
The electromagnetic enhancement for surface enhanced Raman spectroscopy
(SERS) of graphene is studied by inserting a layer of Al2O3 between epitaxial
graphene and Au nanoparticles. Different excitation lasers are utilized to
study the relationship between laser wavelength and SERS. The theoretical
calculation shows that the extinction spectrum of Au nanoparticles is modulated
by the presence of graphene. The experimental results of the relationship
between the excitation laser wavelength and the enhancement factor fit well
with the calculated results. An exponential relationship is observed between
the enhancement factor and the thickness of the spacer layer.
There are few phenomena in condensed matter physics that are defined only by the fundamental constants and do not depend on
material parameters. Examples are the resistivity quantum, h/e2 (h is Planck's constant and e the electron charge), that appears
in a variety of transport experiments and the magnetic flux quantum, h/e, playing an important role in the physics of superconductivity.
By and large, sophisticated facilities and special measurement conditions are required to observe any of these phenomena.
We show that the opacity of suspended graphene is defined solely by the fine structure constant, a = e2/hc � 1/137 (where
c is the speed of light), the parameter that describes coupling between light and relativistic electrons and that is traditionally
associated with quantum electrodynamics rather than materials science. Despite being only one atom thick, graphene is found
to absorb a significant (pa = 2.3%) fraction of incident white light, a consequence of graphene's unique electronic structure.
Raman spectroscopic studies of graphene have attracted much interest. The G-band Raman intensity of a single layer graphene on Si substrate with 300 nm SiO2 capping layer is surprisingly strong and is comparable to that of bulk graphite. To explain this Raman intensity anomaly, we show that in addition to the interference due to multiple reflection of the incident laser, the multiple reflection of the Raman signal inside the graphene layer must be also accounted for. Further studies of the role of SiO2 layer in the enhancement Raman signal of graphene are carried out and an enhancement factor of ~30 is achievable, which is very significant for the Raman studies. Finally, we discuss the potential application of this enhancement effect on other ultra-thin films and nanoflakes and a general selection criterion of capping layer and substrate is given. Comment: 13 pages, 3 figures to be published in Applied Physics Letters
In this report we demonstrate a method for direct determination of the number of layers of hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semiinsulating vanadium-compensated on-axis 6H-SiC(0001). The method anticipates that the intensity of the substrate’s Raman-active longitudinal optical A1 mode at 964 cm^(−1) is attenuated by 2.3 % each time the light passes through a single graphene layer. Normalized to its value in a graphene-free region, the A1 mode relative intensity provides a greatly enhanced topographic image of graphene and points out to the number of its layers within the terraces and step edges, making the technique a reliable diagnostic tool for applied research.
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Graphene has attracted huge attention due to its unique electronic properties, however, when supported those are significantly dependent on the interface interactions. One of the methods of decoupling graphene sheets from a substrate is hydrogen intercalation, which has been shown to produce quasi-free-standing (QFS) layers on a SiC (0001) surface. Still, the effects of incomplete H termination of SiC remain mostly unknown. This work in vestigates, employing density functional theory calculations, the impact of partial termination on the structural, and electronic properties of graphene. It is predicted that interfaces with partially damaged H layer or produced under a lower technological standard could still benefit from the intrinsic, however, quantitatively reduced, properties of QFS graphene.
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The nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM), conductive-atomic force microscopy (C-AFM) and Raman spectroscopy. Differently than for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by Al2O3 island growth in the very early stages, followed by the formation of a continuous Al2O3 film (∼2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. The electrical insulating properties of the deposited ultrathin Al2O3 films were demonstrated by nanoscale current mapping with C-AFM. Raman spectroscopy analyses showed low impact of the ALD process on the defect’s density of EG. The EG strain was also almost unaffected by the deposition in the regime of island growth and coalescence, whereas a significant increase was observed after the formation of a compact Al2O3 film. The obtained results can have important implications for device applications of epitaxial graphene requiring ultra-thin high-k insulators.
Raman spectroscopy in combination with appropriate sample preparation strategies, for example enrichment of bacteria on metal surfaces, has been proven to be a promising approach for rapidly diagnosing infectious diseases. Unfortunately, the fabrication of the required chip substrates is usually very challenging due to the lack of feasible instruments that can be used for quality control in the surface modification process. The intrinsically weak Raman signal of the biomolecules, employed for the enrichment of the microorganisms on the chip surface, does not allow monitoring the successful immobilization by means of a Raman spectroscopic approach. Within this contribution we demonstrate how a simple modification of a plain aluminum surface enables enhancing (or decreasing, if desired) the Raman signal of molecules deposited on that surface. The manipulation of the Raman signal strength is achieved via exploiting interference effects that occur, if the highly reflective aluminum surface is modified with thin layers of transparent dielectrics like aluminum oxide. The thicknesses of these layers were determined by theoretical considerations and calculations. For the first time it is shown that the interference effects can be used for the detection of biomolecules as well by investigating the siderophore ferrioxamine B. The observed degree of enhancement was approximately one order of magnitude. Moreover, the employed aluminum/aluminum oxide layers have been thoroughly characterized using atomic force and scanning electron microscopy as well as X-ray reflectometry and UV-Vis measurements.
We demonstrate the preparation of a controllable and reproducible active substrate for surface enhanced Raman scattering (SERS) using a facile oxidation method that allows us to obtain a titanium oxide (TiO2) capping layer with desired thickness on nickel-titanium alloy (NiTi). The carefully tuned oxide layer, which is obtained by controlling the annealing time,exhibits the enhancement of 2D band intensity of graphene up to ~ 50 times in comparison to bare nitinol The dependence of Raman enhancement with the oxide thickness can be well explained by the interference enhanced Raman scattering (IERS) process, and fitted to a multi reflection model (MRM) of the Raman scattering of graphene on layered structure. Thus our results provide a facile method to enhance Raman signals of graphene by tuning the thickness of the oxide layer at all three lasers (514 nm, 633 nm and 785 nm). The present method can be adapted to exploit the recent advances in molecular vibration study and biomolecular detection due to the versatility of the proposed substrate.
The work presented here describes the first steps toward designing a thermally robust surface-enhanced Raman spectroscopy (SERS) substrate with the potential to conduct in situ monitoring of catalytic reactions. Nanosphere lithography (NSL) fabricated SERS substrates were coated with thin (0.2-1.0 nm) films of atomic layer deposited (ALD) Al 2O 3. The thermal stability of these substrates was examined at various temperatures (100-500°C) and over time (up to 6 h) in nitrogen. The results showed that ALD Al 2O 3 coated nanoparticles maintained their original geometry significantly better than the bare Ag nanoparticles. While experiments showed that thicker ALD Al 2O 3 coatings resulted in the most stable nanoparticle structure, ALD Al 2O 3 coatings as thin as 0.2 nm resulted in thermally robust nanostructures as well. Additionally, the ALD Al 2O 3 coated nanoparticles were heated under propane to mimic reaction conditions. These experiments showed that while the nanoparticle geometries were not as stable under reducing atmosphere conditions, they were much more stable than uncoated nanoparticles and therefore have the potential to be used for SERS monitoring of reactions conducted at elevated temperatures.
In this study, we developed a reliable method to analyze the interference-enhanced Raman scattering (IERS) effect on graphene by considering the surface electric field (E-field), which can be calculated precisely by measuring the optical admittance of the thin-film assembly. Through accurate tuning of the optical properties of one-dimensional photonic crystals (1D-PhCs), the strong and controllable interference effect allowed the surface E-field to be maximized and, thereby, to optimize the enhancement factors of the Raman scattering signals of graphene. Using this approach, we could enhance both the G and the 2D bands of graphene largely, uniformly, and equally, by about 180 times relative to those obtained on a silicon substrate. Under certain conditions, the Raman peak of graphene could even be enhanced by over 400 times. After transferring single-layer graphene (SLG) and few-layer graphene (FLG) onto various substrates, we found that the Raman spectra of both SLG and FLG on the 1D-PhCs substrate were enhanced without changing the band-to-band ratio or the peak positions of the main Raman bands of graphene. Without inducing any additional signal disturbance, this enhancement technique allowed us to maintain the accurate and precise informational features from the Raman spectra. The experimental enhancement factors in the coenhanced Raman spectra of graphene were higher than those previously obtained using the IERS effect. Moreover, the surface E-field of 1D-PhCs could be modulated by changing the incident angle of the excited light source, thereby allowing fine-tuning of the working wavelength. Thus, by controlling only the surface E-field, the Raman signals of graphene could be enhanced dramatically without any distortion on spectra. Accordingly, using 1D-PhCs and the optimized IERS effect is very helpful for fine structural characterization of graphene through conventional Raman spectroscopy.
In situ probing of surface species and incipient phases is vital to unraveling the mechanisms of chemical and energy transformation processes. Here we report Ag nanoparticles coated with a thin-film SiO2 shell that demonstrate excellent thermal robustness and chemical stability for surface enhanced Raman spectroscopy (SERS) study of solid oxide fuel cell materials under in situ conditions (at 400 °C).
The Raman spectra of very thin evaporated films of metallic titanium and oxidized titanium are obtained with a new technique called interference-enhanced Raman scattering. The results indicate that titanium films exhibit a crystalline hcp structure while the titanium oxide has an amorphous structure with local atomic bonding configurations similar to those in crystalline Ti2O3.
Surface-enhanced Raman spectroscopy (SERS) was used to monitor the response of a self-assembled monolayer (SAM) of a tetrathiafulvalene (TTF) derivative on a gold film-over-nanosphere electrode. The electrochemical response observed was rationalized in terms of the interactions between TTF moieties as the oxidation state was changed. Electrochemical oxidation to form the monocation caused the absorbance of the TTF unit to coincide with both the laser excitation wavelength and the localized surface plasmon resonance (LSPR), resulting in surface-enhanced resonance Raman scattering (SERRS). The vibrational frequency changes that accompany electron transfer afford a high-contrast mechanism that can be used to determine the oxidation state of the TTF unit in an unambiguous manner.Keywords: rotaxanes; catenanes; tetrathiafulvalene; SERS; LSPR; spectroelectrochemistry; SAM
A new method of obtaining Raman spectra from a very thin highly absorbing films (α≳105 cm-1) is described. The technique which is termed interference enhanced Raman scattering (IERS) is shown theoretically to produce a gain in the scattered intensity of 10–103 (depending on the optical constants of the material) over that expected from a thick sample using the conventional Raman backscattering configuration. The potential of the method is demonstrated experimentally using tellurium, and a gain of 20 is obtained.
In this work we have verified the remarkable sensitivity of Raman spectroscopy for the study of adsorbed pyridine on a silver surface, and extended its applicability to other nitrogen heterocycles and amines. New bands in the scattering spectrum of adsorbed pyridine have been characterized, which were not previously reported, as well as the Raman intensity response of all the surface pyridine bands as a function of electrode potential. As a result of these experiments, we have proposed a model of the adsorbed species for pyridine in which the adsorption is anion induced, leading to an axial end-on attachment to the electrode surface. The ability to obtain resonance Raman spectra with good signal-to-noise with laser powers less than 1.0 mW, reported here for the first time, opens up possibilities of surface Raman studies with relatively inexpensive laser systems. As laser power requirements are relaxed, reliability is improved, and greater tuning ranges can be achieved for wavelength dependent studies. We previously demonstrated the potential of resonance Raman spectroscopy for monitoring solution kinetic behavior [2], and now have shown that NR as well as RR spectroscopy has sufficient sensitivity to extend the studies of kinetic processes to include those occurring at electrode surfaces.
Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.
We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.
Insight into the nature of transient reaction intermediates and mechanistic pathways involved in heterogeneously catalyzed chemical reactions is obtainable from a number of surface spectroscopic techniques. Carrying out these investigations under actual reaction conditions is preferred but remains challenging, especially for catalytic reactions that occur in water. Here, we report the direct spectroscopic study of the catalytic hydrodechlorination of 1,1-dichloroethene in H2O using surface-enhanced Raman spectroscopy (SERS). With Pd islands grown on Au nanoshell films, this reaction can be followed in situ using SERS, exploiting the high enhancements and large active area of Au nanoshell SERS substrates, the transparency of Raman spectroscopy to aqueous solvents, and the catalytic activity enhancement of Pd by the underlying Au metal. The formation and subsequent transformation of several adsorbate species was observed. These results provide the first direct evidence of the room-temperature catalytic hydrodechlorination of a chlorinated solvent, a potentially important pathway for groundwater cleanup, as a sequence of dechlorination and hydrogenation steps. More broadly, the results highlight the exciting prospects of studying catalytic processes in water in situ, like those involved in biomass conversion and proton-exchange membrane fuel cells.
We propose a novel surface and interference coenhanced Raman scattering technique to dramatically enhance the Raman signal intensity of graphene by using a specifically designed substrate of Si capped with surface-active metal and oxide double layers (SMO). The total enhancement ratio can reach the order of 10(3) compared with the original Si substrate. Combining the visibility of graphene on the SMO substrate, we demonstrate that the tiny structure change and surface structure of graphene can be easily detected. This technique makes Raman spectroscopy a more powerful tool in the field of ultrasensitive characterization of graphene, isolated carbon nanotubes, and other film-like materials.
Time, cost, and casualties associated with demining efforts underscore the need for improved detection techniques. Reduction in the number of false positives by directly detecting the explosive material, rather than casing material, is desirable. The desired field sensor must, at a minimum, demonstrate reproducibility, the necessary level of sensitivity, portability, instrumental stability, and fast system response times. Ideally, vibrational spectroscopic techniques have the potential to remove false positives, since every chemical has a unique bond structure. Herein, we demonstrate the capabilities of surface-enhanced Raman spectroscopy to detect the chemical vapor signature emanating from buried TNT-based landmines. We present reproducible results obtained from blind tests controlled by the Defense Advanced Research Projects Agency (DARPA) that demonstrate vapor detection of 2,4-dinitrotoluene at concentration levels of 5 ppb or less. The results presented used acquisition times of 30 s on a fieldable system and showed that SERS can be a significant improvement over current landmine detection methods.
We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions,
metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between
valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in
concentrations up to 1013 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by
applying gate voltage.
Surface Raman spectroelectrochemistry