Nanoalloy libraries from laser-induced thermionic emission reduction.

Nanoalloys, especially high-entropy nanoalloys (HENAs) that contain equal stoichiometric metallic elements in each nanoparticle, are widely used in vast applications. Currently, the synthesis of HENAs is challenged by slow reaction kinetics that leads to phase segregation, sophisticated pretreatment of precursors, and inert conditions that preclude scalable fabrication of HENAs. Here, we report direct conversion of metal salts to ultrafine HENAs on carbonaceous support by nanosecond pulsed laser under atmospheric conditions. Because of the unique laser-induced thermionic emission and etch on carbon, the reduced metal elements were gathered to ultrafine HENAs and stabilized by defective carbon support. This scalable, facile, and low-cost method overcomes the immiscible issue and can produce various HENAs uniformly with a size of 1 to 3 nanometers and metal elements up to 11 with productivity up to 7 grams per hour. One of the senary HENAs exhibited excellent catalytic performance in oxygen reduction reaction, manifesting great potential in practical applications.

INTRODUCTION

Metal nanoalloys are critical catalysts and widely used in many chemical reactions ranging from environmental to energy fields (–). Conventional bottom-up routes, such as wet-chemistry methods for the synthesis of metal nanoalloys, usually need to consider the miscibility of each metallic element in the phase diagrams to avoid the phase segregation during the formation of the particles (–). The immiscible issue of metals restricts the realm of nanoalloys and precludes the discovery of new nanoalloys with superior functionalities (–). In recent years, high-entropy nanoalloys (HENAs) containing equal stoichiometric ratios of various metals in each particle in a well-mixed manner have attracted a lot of interest because of their unusual physical and chemical properties. These unique properties make them attractive as catalysts (–), such as oxygen reduction reaction (ORR) that has great potential for applications in energy and environmental science. Recent studies have revealed that the slow reaction kinetics in traditional methods is one of the major obstacles that lead to the phase segregation in nanoalloys (, ). To tackle this challenge, fast synthetic methods, such as carbothermal, fast-moving bed heating, pulsed electrochemical reduction, electrical sparkling, laser ablation, and microwave heating have been developed by reduction of the metal ions in the time range from milliseconds to seconds (, –). However, sophisticated procedures for the preparation precursor, aggregation issue of HENAs, and the inert atmosphere requirement still restrict the scalable production of high-quality HENAs.

Here, we report the direct fabrication of supported ultrafine HENAs in air via nanosecond pulsed laser reduction of metal salts on carbonaceous support under atmospheric conditions. Carbonaceous materials, such as graphene and carbon nanotubes (CNTs), are prone to be excited by laser for thermionic emission (–), during which notable electron emission is ejected out from the graphene (Fig. 1A). When laser photons are absorbed by graphene, electrons from the valence band were excited to the conduction band, and population inversion is achieved and maintained. Then, hot electrons obtained with enough energy will be ejected out from graphene and become free electrons through Auger-like pathways (). These free electrons would act as a reductant for the metal ions. In addition, the electron flow also propels the precursor powder across the container (Fig. 1B), thus achieving an even and homogeneous reaction. This ultrafast laser reaction precludes the phase separation of alloys and is capable of synthesizing libraries of alloys with a good dispersion on the supports. This method is straightforward and convenient compared with previous methods where sophisticated precursors preparation, inert reaction atmosphere, and high energy cost are necessary.

Fig. 1.

The LITER for the synthesis of nanoalloys.

(A) Illustration of the laser-induced thermionic emission in graphene. Four steps were divided in this process: (1) The laser photons excite electrons from the valence band to the conduction band; (2) a population inversion state is achieved; (3) the Auger-like pathways of electrons; and (4) some hot electrons gain enough energy and eject out as free electrons. (B)The schematic of the laser propulsion of graphene nanoplates across a glass vial that achieved evenly irradiation and reduction of the metal salts loaded on graphene. (C) The optical images of the precursor on the glass vial when the laser is on and off. (D) The illustration of laser-induced electron emission on graphene with metal ions loaded on the surface. (E) The four steps of the LITER process for the formation of ultrafine nanoalloys on carbonaceous supports. Balls with different colors represent different metal ions or atoms.

The laser energy packages with a pulse duration of 5 ns and a pulse energy up to 600 mJ could be precisely delivered to the carbonaceous supports and generate an obvious plasma plume containing electron jet flow (Fig. 1, C and D). There are three steps for the formation of the HENAs by laser (Fig. 1E). At first, the laser photons were absorbed by the carbonaceous support and the metals ions, then electrons were generated, and high-temperature conditions were achieved to initiate the reduction and etching of the carbonaceous support. The laser-induced plume with localized high temperature and reductive atmosphere prevented the oxidation issue, allowing the whole process applicable in air. Last, the reduced metal atoms were instantly cooled after laser irradiation and gathered into ultrafine nanoalloys on the defect sites of the carbon support (Fig. 1E). This laser-induced ultrafast reduction and cooling in nanoseconds enable the rapid reduction of various metal salts to form HENAs with uniform sizes and even distribution on the supports. The noncontact laser interaction also makes this method suitable for the processing of powdery precursors, which was compatible with modern industry fabrication and would markedly reduce the cost. We termed this method as laser-induced thermionic emission reduction, LITER for short, and used this method to fabricate HENAs with elements up to 11. We found that the HENAs stabilized on few-layered graphene synthesized by scalable LITER fabrication exhibited high catalytic activity in ORR, manifesting the potential of this method to produce HENAs in practical applications.

RESULTS AND DISCUSSIONS

The operation of the LITER method includes two steps, loading of metal salts on carbonaceous supports to form the precursor and subsequent laser treatment of the precursor (fig. S1). Here, we used the fabrication of few-layered graphene supported HENAs as an example to demonstrate the advantage of this method. First, a few-layered graphene powder was dispersed in the ethanol solvent containing corresponding chloride metal salts under stirring. Then, the ethanol solvent was evaporated under vacuum to obtain the graphene-supported metal salt precursor. At last, the precursor was loaded in a glass vial and subjected to nanosecond laser pulses in air (Fig. 1A). Typically, the spot size of the laser pulses was 5 mm, and the pulse energy was 620 mJ. A quartz glass was placed on the opening of the vial to prevent the loss of precursor during the laser treatment. As laser pulses interacted with the precursors, high-density plasma plume was formed, and the graphene flakes were propelled across the whole bottle container. Upon laser shock, the graphene layer could absorb the laser pulse and converted the energy to heat, thus forming a high-temperature local environment to pyrolysis the metal salts simultaneously. During the laser shock, a flicker occurred when the laser pulse was delivered onto the graphene target (Fig. 1, B and C), suggesting a high temperature formed in this process. Since laser could be dissipated as heat in nanosecond time scale through lattice vibration, the 5-ns laser energy package could be precisely delivered to a local area to create a high-temperature environment. After laser exposing, the metal salts were rapidly decomposed to form metal atoms and mixed uniformly and facilitate the formation of HENAs without phase separation.

Before the synthesis of HENAs, we used LITER to synthesize ultrafine Pt nanoparticles decorated on few-layered graphene to investigate the capability of laser reduction under atmospheric conditions. The precursor was prepared by wet impregnation of PtCl4 salt on the surface of few-layered graphene and then dried under vacuum to obtain a black powder. The black powdery precursor was loaded in a glass vial, and a glass cover was placed on the opening of the vial to prevent leakage of a product during the laser treatment. A laser pulse with a pulse energy of 620 mJ, a pulse duration of 5 ns, a spot size of 5 mm, and a wavelength of 1064 nm was delivered to the powdery precursor through the glass cover to initiate the reduction of the metal salts. In a typical trial, the laser dose used for the precursor was 620 mJ mg−1. When the laser pulses interacted with the carbonaceous precursor, obvious light emission was observed from the glass vial (Fig. 1C), suggesting the formation of a plasma plume during the laser treatment. We also noticed the rise of “black smoke” on the vial when the laser was delivered on the precursor (fig. S2), ascribing to the laser-induced propelling of the graphene by strong electron emission with laser (). To confirm the presence of electron flow, a homemade device was made by detecting the current in a sealed cell (fig. S3). Obvious current response up to 30 μA was detected under vacuum and 14 μA under ambient conditions. As the graphene flakes were small (~10 μm), the powdery precursor would be subject to sufficient movement across the closed vial and received even laser irradiation (Fig. 1B). After laser irradiation, the obtained black powder was soaked in fresh ethanol for 24 hours to dissolve unreacted metal salts and dried under vacuum.

The obtained product was first characterized by microscopy to reveal the structure. Scanning electron microscopy (SEM) images showed that the obtained product was identical to the pristine few-layered graphene without the presence of any particle (fig. S4). We assumed that the size of the obtained metal nanoparticles (MNPs) was ultrasmall that was beyond the distinguishability of SEM. We then used transmission electron microscopy (TEM) to reveal the morphology of the product. Uniform and evenly distributed white species, with higher atomic numbers, were observed in the high-angle annular dark-field (HAADF) images (Fig. 2A), which is different from the dark background of the graphene support. The magnified image showed that uniform nanoparticles were formed on graphene (Fig. 2B), and these nanoparticles exhibited identical selected-area electron diffraction (SAED) patterns as face-centered cubic (fcc) Pt particles (Fig. 2C). Statistical data showed that the particle sizes of the synthesized Pt nanoparticles centered around 2 nm (Fig. 2D). The large-area TEM images also uncovered that the Pt nanoclusters were uniformly distributed across the whole graphene layer with a high density of 106 mm−2 (fig. S5), indicating the uniform conversion of the metal salts on graphene induced by laser pulses. As supported by the defective sites of graphene, these Pt nanoparticles exhibited high stability to resist aggregation under thermal annealing at 400°C for hours (fig. S6).

Fig. 2.

TEM characterization of nanoalloys.

(A and B) The TEM images of Pt nanoparticles fabricated by the LITER method. (C) The SAED pattern of Pt nanoparticles on graphene. (D) The particle size distribution of Pt nanoparticles. (E) TEM image of PtPdNi nanoparticles on graphene and the corresponding (F) elemental mappings, (G) SAED pattern, and (H) particle-size distribution plot. (I) High-resolution TEM image of PtPdCoNi nanoalloys on graphene and the corresponding (J) SAED pattern and (K) particle-size distribution plot. (L) High-resolution TEM image of PtPdCoNiCuAuSnFe nanoalloys on graphene and the corresponding (M) SAED pattern and (N) particle-size distribution plot. a.u., arbitrary units.

Similar to the synthesis of Pt nanoparticles, LITER is general for the fabrication of a large variety of nanoalloys on graphene by loading designated metal salts in the precursor. The binary HAADF images revealed that the PtPdNi nanoalloys by LITER exhibited excellent uniformity in particle size and distribution (Fig. 2E). The elemental mappings from energy dispersive spectroscopy (EDS) revealed identical signals from Pt, Pd, and Ni on the carbonaceous support (Fig. 2F), suggesting the successful fabrication of ternary PtPdNi nanoparticles on graphene. SAED pattern also revealed that the PtPdNi nanoparticles exhibited a similar fcc crystalline structure as Pt and gave an average particle size of 2.2 nm (Fig. 2, G and H). We further synthesized quaternary (PtPdNiCo), senary (PtPdNiCoCuAu), and octonary (PtPdNiCoCuAuFeSn) nanoalloys by LITER method, and they all showed ultrafine sizes, uniform distribution, and well-matched elemental mappings across the graphene support (figs. S7 to S13). To further confirm the well-mixed nature of different elements in each MNP, high-resolution spherical aberration–corrected TEM was performed. According to the difference in atomic number, we found that atoms with a lower atomic number were randomly mixed with those with higher atomic number and these metallic atoms aligned to ordered fcc lattice facet with an interplanar distance of 0.26 nm (Fig. 2, I, J, L, and M), implying that no phase segregation occurred and confirming the high-entropy mixing of elements (). These HENAs also show an even particle size of around 2 nm (Fig. 2, K and N), smaller than HENAs synthesized by previous studies with typical sizes from 10 to 100 nm (–).

To give more convincing evidence of the well-mixed elements in each nanoparticle, we synthesized PtAuRhIrSn nanoalloys with a larger particle size of around 15 nm for energy-dispersive x-ray (EDX) mapping. These quinary nanoalloys were synthesized by the LITER method with much more laser pulses, and they also dispersed uniformly on graphene substrate (Fig. 3A). The EDS mappings on the whole graphene flake clearly showed that all metallic elements exhibited uniform distribution on graphene. The detailed EDS mappings of the quinary nanoalloys of several particles also confirm that metallic elements in each nanoparticle were well mixed (Fig. 3B), which was consistent with the EDS line scanning spectra (fig. S14). To explore the capability of LITER method for the fabrication of HENAs, we prepared a precursor with 11 metallic elements for the fabrication of FeCoNiCuPtRhPdAgSnIrAu nanoalloys. We found that all metallic elements showed identical distribution across each nanoparticle (Fig. 3C), confirming the successful fabrication of the HENAs across a large variety of metals. Noting that some of the metal elements were immiscible with each other, Au versus Ni, Cu versus Sn, and Pd versus Fe, for example, LITER provides a general tool to tackle the immiscible issue in nanoalloy fabrication via ultrafast reduction of the precursors in nanoseconds, precluding the immigration of atoms that leads to the phase separation (). Powder x-ray diffraction (PXRD) revealed that only 111 lattice facet assigned to fcc crystals was observed (Fig. 4A), implying that no phase segregation was found in the HENAs fabricated by LITER.

Fig. 3.

The elemental distribution analysis of the HENAs.

(A) The HAADF image of PtAuRhIrSn HENAs on graphene and the corresponding elemental mappings in large area. (B) Well-matched elemental mappings in PtAuRhIrSn HENAs. PXRD patterns of the pristine ZIF-8 nanocrystals laser shock processed ZIF-8 blocks. (C) The HAADF image of HENAs with 11 elements (FeCoNiCuPtRhPdAgSnIrAu) on graphene and the corresponding elemental mappings.

Fig. 4.

The characterization of the HENAs and graphene support.

(A) The PXRD patterns of different HENAs obtained by LITER method. (B) The Raman spectra of graphene, laser-treated graphene, and laser-treated graphene with metal salt precursors on them.

The stoichiometric ratio and chemical state of the element in HENAs were further analyzed. The EDX analysis of the HENAs with 11 elements showed that the atomic ratio of each element varied with each other when fed with a similar molar ratio of each element (table S1). The inductively coupled plasma–atomic emission spectrometry also revealed the difference in atomic ratio, from 2.7 to 15.4% (table S2). We found that the conversion of the metal ions in the precursor varies with different metals. For example, Pt4+ ions are more prone to be reduced than the Ni2+ ions as analyzed by the element contents in the nanoalloys (fig. S15). This difference in the stoichiometric ratio in the alloys might be ascribed to their intrinsic difference in their reduction potentials (). Elements with higher reduction potentials are prone to be reduced and occupy higher ratios in the alloys (tables S3 to S10). The atomic ratio in the HENAs could be adjusted by tuning the feeding stoichiometric ratio of the metal salts in the precursor. For example, when the feeding molar ratio in the precursor was Pt:Pd:Rh:Co:Ni = 2:2:2:3:3, the obtained senary PtPdRhCoNi nanoalloy exhibited an equivalent molar ratio in each element (table S10). Furthermore, x-ray photoelectron spectroscopy (XPS) was used to reveal the chemical state of the elements in the HENAs fabricated by LITER. We found that noble metals were in a zero-valent state, while transition metals, such as Sn and Cu, were partially oxidized because of their higher activity (fig. S16) ().

Previous studies have revealed that graphene and CNTs are excellent “hot carriers” emitters (–). It is well known that graphene and CNTs have unique optoelectronic properties due to their Dirac conical and gapless band structure (, ). This allows them to absorb the laser photons efficiently and easily achieve a population inversion state because of the excitation of hot electrons and the bottleneck of the relaxation at the Dirac point. Electrons with enough energy would eject as hot electrons following a proposed Auger-like mechanism (Fig. 1A) (). The ejected hot electrons not only act as a reductant for the metal salts but also propel the precursor powder across the container (Fig. 1, B and D, and fig. S2), so all metal salts would be reduced evenly to ultrafine HENAs after being subjected to multiple laser pulses. Another advantage of LITER is the self-etching induced defects on graphene/CNTs during laser treatment of the precursor. The Raman spectrums carried out on few-layered graphene, laser-processed graphene, and laser processed graphene with metal salts on the surface showed that high-density defects are prone to be formed in the presence of metal salts on graphene (Fig. 4B). Almost no obvious change in the relative intensity between D band and G band (ID/IG) before and after nanosecond laser irradiation on pure graphene (Fig. 4B and fig. S17), suggesting that no gain in defects in the laser-processed graphene (). However, when metal salts were added to graphene, a obvious gain in ID/IG was observed. We also found that Raman peaks assigned to metal salts disappeared after LITER processing, also confirming the reduction of metal salts to metals (fig. S18). The rich defects on graphene would act as anchor sites for the formed HENAs and stabilized them to resist the aggregation effects. The rich defects found on graphene by LITER method were consistent with the former phenomenon that graphene was prone to be etched in the presence of metal atoms on the surface (, ). Our LITER method not only successfully reduced the metal salts into ultrafine HENAs but also simultaneously created rich defects on the carbon support for the stabilization of the HENAs. Given the facile precursor preparation and straightforward laser processing, the LITER method stands out from previous joule heating and electrochemical methods. The LITER method is also compatible with the continuous manufacturing process in the modern industry without the involvement of vacuum and inert gas protection. Compared with previous methods, LITER is superior in reducing time scale and particle sizes (fig. S19), which are important for practical production and application.

To demonstrate the potential applications of the synthesized HENAs, we carried out electrochemical performance analysis by fabricating HENAs on CNTs (figs. S20 to S22). A conventional rotating disk electrode setup was used to evaluate the catalytic performance. As shown in Fig. 5A, the PtPdRhFeCoNi sample exhibited an oxygen reduction peak at 0.65 V versus reversible hydrogen electrode (RHE) in the cyclic voltammetry (CV) test after the first activation cycle. To probe the ORR catalytic activities, linear sweep voltammetry (LSV) measurements were performed in O2-saturated 0.1 M KOH solution with different rotating speeds from 400 to 2500 rpm. All samples exhibited similar half-wave potentials (E1/2) of around 0.85 V versus RHE (Fig. 5B), indicating superior ORR catalytic activity. No obvious difference was found from the polarization plots when graphite rod was used as the counter electrode, excluding the impact of Pt leaching from the Pt electrode (fig. S23). We noticed that the limited current density (jlim) could be gradually improved by altering the elements in the alloys. For example, the values of jlimit in PtPdRh and PtPdRhCo were determined as 4.21 to 4.39 mA cm−2, respectively, and increased to 4.89 mA cm−2 in PtPdRhFeCoNi (Fig. 5C). We noticed that some of the HENA catalysts showed lower activity than the commercial Pt/C counterpart, while others outperformed the Pt/C catalyst. Pt/CNT synthesized by LITER method exhibited better performance than the Pt/C catalyst when the Pt contents, 20 weight %, were the same. The relatively lower atomic ratio of noble metals in the catalyst still makes HENAs attractive for the synthesis of low-cost catalysts. As the LITER method provides the possibility for the ultrafast synthesis of HENAs with wide choices of elements and their ratios, we believe that rational screening of the HENA by computer or other methods will lead to the discovery of advanced catalysts with better performance (, ).

Fig. 5.

The electrocatalytic performance of the HENAs in ORR.

(A) The CV curves and (B) the ORR polarization plots under different rotation speeds of HENA catalyst of PtPdRhFeCoNi on CNTs. (C) ORR polarization plots of different catalysts measured at speed of 1600 rpm. (D) The electron transfer number of PtPdRhFeCoNi on CNTs derived from Koutecky-Levich plots at a potential of 0.4 V versus RHE.

The HENA-based catalyst performed much better performance than the standard Pt/C electrodes and exhibited excellent stability after cycling for 5000 times (fig. S24). In addition, the TEM and XPS analysis of the cycled catalyst revealed that the HENA catalysts still maintained their ultrasmall size without obvious aggregation and their metallic nature was also unaltered (figs. S25 and S26). The electrochemical surface area (ESCA) of the HENA catalyst was calculated to be 86 m2 gPt−1 (fig. S27), excelling the well-established Pt/C catalysts with ESCA around 70 m2 gPt−1 (, –). The HENA catalyst could maintain the high activity at a long-term test without degradation in the current density and half-wave potentials (fig. S28). The Pt/C and Pt/CNT also showed high stability under a long-term CV scan (fig. S29); however, Pt/CNT synthesized from LITER exhibited a much higher current density. To gain deeper insights into the electron transfer pathway over PtPdRhFeCoNi, LSV tests at various rotating speeds of 400 to 2500 rpm were carried out. As shown in Fig. 5D, the number of transferred electrons calculated on the basis of the Koutecky-Levich analysis was 3.92, indicating that PtPdRhFeCoNi tends to reduce oxygen molecules via a four-electron transfer step in ORR (fig. S30). Other HENA catalysts also exhibited a four-electron transfer behavior in the ORR reaction (fig. S31). The above analysis evidenced the excellent ORR activity of HENAs obtained by LITER.

In summary, we reported the facile refine of uniform HENAs from their corresponding metal salt precursor under ambient conditions by direct laser-induced thermionic emission on graphene and CNTs in nanoseconds. The unique electron emission induced reduction and simultaneously formed defects on the carbonaceous support instantly reduced the metal ions to ultrafine HENAs stabilized on the defect sites. These HENA nanoclusters with the size down to several nanometers delivered a remarkable catalytic performance in ORR. This LITER method is also advanced in mixing various elements into ultrasmall alloys in a scalable and energy-efficient manner (fig. S32). Given the rich combination of elements, the ultrafast laser technology, and the scalable feature, this method opens a new way to produce alloy libraries with unique properties to meet the needs in other energy and environmental applications.

MATERIALS AND METHODS

Materials

Few-layered graphene and single-wall CNTs were purchased from Xianfeng Nano Ltd. Co. and used without further purification. Metal salts, CuCl2·2H2O (99.95%), NiCl2·6H2O (99.9%), CoCl2·6H2O (99.9%), FeCl3·6H2O (99.9%), SnCl4·5H2O (98%), PtCl4 (99.9%), PdCl2 (99.9%), IrCl4·xH2O (99.9%), and HAuCl3·3H2O (99.9%) were purchased from Sigma-Aldrich and directly used without further purification.

Sample preparation

The precursor prepared for laser treatment was obtained by wet impregnation of the corresponding metal salts on the carbons supports and subsequent vacuum dry. Specifically, metal salts were dissolved in ethanol by sonication bath and then added to a glass vial with a specific volume to make a mixed solution. Then, the carbonaceous powder, such as graphene or CNTs, was added and evenly dispersed in the above solution under a sonication bath for 20 min. The obtained dispersion solution was then dried and degassed under a vacuum at room temperature overnight. Last, the powdery precursor was obtained and sealed to prevent moisture. In typical laser treatment, the glass vial containing the precursor was placed under the focus lens. The laser pulses (Nd:YAG, Surelite III Q-switch, Continuum, wavelength = 1064 nm, pulse duration = 5 ns) were delivered to the precursors automatically with a frequency of 2 Hz. Once the laser contacted the precursor, a bright light was observed and, along with floated black smoke, saturated across the whole vial. The light was caused by the plasma plume, and the black smoke was generated by the powdery precursor thrusted by the ejection of electron flow and plasma from the precursor during laser irradiation. In a typical trial for the synthesis of HENAs with ultrasmall sizes, the laser irradiation dose was 1 pulse per milligram of the precursor. For HENAs with larger sizes, the laser pulses delivered to the precursors increased gradually to 10-fold as much as that in the production of small-sized HENAs. After laser treatment, the obtained product was soaked three times in fresh ethanol to dissolve residual metal salts. The final product was obtained by vacuum dry overnight at room temperature.

Characterization

SEM images were collected from a Thermo Fisher Scientific Teneo Volumescope SEM equipped with a field emission gun, and trinity detector operated at 10 kV, the ion source was operated at 30 kV. High-resolution XPS was performed on ESCALAB 250Xi (Thermo Fisher Scientific) with Al Kα as an excitation source. The PXRD patterns were collected from a Panalytical Empyrean PXRD under Bragg-Brentano mode using Cu Kα radiation (λ = 1.54178 Å), operated at 40 kV and 40 mA. Data were collected between 15° and 40° in 2θ using the Panalytical Data Collector software. TEM images, selected-area diffraction patterns, and EDS under scanning TEM (STEM) mode were analyzed by using a Thermo Fisher Scientific Talos 200X TEM equipped with a Super-X EDS system, operated at 200 kV. The atomic resolution STEM images were obtained from Thermo Fisher Scientific Themis Z aberration-corrected TEM operated at 300 kV. Samples obtained under different pulse energy were measured by Raman spectra with a WiTEC AFM/Raman system at room temperature with a laser excitation wavelength of 632.8 nm.