Surface-Enhanced Raman Scattering on a Chemically Etched ZnSeSurfaceSyed K. Islam, Maria Tamargo, Richard Moug, and John R. Lombardi*

Department of Chemistry and Center for Exploitation of Nanostructures in Sensor and Energy Systems (CENSES), The City Collegeof New York, 160 Convent Avenue, New York 10031, United States

ABSTRACT: We report the observation of surface-enhanced Raman scattering (SERS) from achemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin filmof ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid.Protrusions of hemiellipsoidal nanoparticles are observed on the surface. Using the results of theMie theory, we controlled the size of the nanoparticles to overlap significantly with maximumefficiency of near-field plasmon enhancement. In the Raman spectrum, we observe largeenhancements of the a1, b1, and b2 modes when 4-MPy molecules are adsorbed on the surfaceusing a 514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2 × 106) isobserved comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupledcontribution of several resonances. We propose that some combination of surface plasmon, charge transfer, and band-gapresonances is most likely the contributing factor in the observed Raman signal enhancement because all three of these resonanceslie close to the excitation wavelength.

■ INTRODUCTIONThe proximity of a metal nanoparticle to a molecule has beenfound to considerably enhance the Raman intensity1 by factorsin the range of 106 to 1012. This phenomenon, known assurface-enhanced Raman spectroscopy (SERS), has beenshown to be caused by several resonances in the molecule−metal system, including the surface plasmon of the metalconduction band, various molecular resonances, as well ascharge-transfer resonance between the molecule and nano-particle.2 Recently, a considerable amount of attention has beenfocused on extending SERS to semiconductor substrates. Inmost semiconductor nanoparticles, the surface plasmonresonance due to the conduction band lies typically in theinfrared. This is because of a very low electron density in theconduction band. Thus any enhancement of Raman signal isusually attributed solely to charge transfer, and enhancementfactors somewhat lower than those observed on metals havebeen obtained. SERS has been reported on ZnO, ZnS, PbS,CdS, TiO2, CdSe/ZnBeSe, CdTe, Se nanowires, and Genanotubes using various molecules as probes.3−10 The reportedenhancement factor in these semiconductors ranges between102 and 105.However, charge transfer is not the only factor attributed to

SERS in semiconductors; in addition to plasmons originating inthe conduction band, the valence band (VB) can also supportplasmons, and these are usually observed in the ultraviolet.Such observations are discussed in detail by Pines.11,12 Theelectron density of valence electrons in solids is typicallyaround 1022 to 1024 cm−3, so that the resulting bulk plasmonexcitation energy is expected to be found in the ultraviolet.These resonances can be shifted to lower energies by varyingthe size and shape of semiconductor nanoparticles. Hayashi etal. showed dependence efficiency of near-field enhancement(Qnf) on the size of spherical GaP particles.13 Using Mie

scattering theory, they found that particles in the range of 100−140 nm diameter could support a plasmon resonance in thevisible region of the spectrum (514.5 nm). The authorsindicated an enhancement factor of as much as 106 usingcopper phthalocyanine as a probe molecule. Quagliano et al.reported SERS on etched GaAs surface using pyridinemolecules due to EM enhancement.14 Recently, SERS onsmall Cu2O nanoparticles and on 3D TiO2 nanowires havebeen shown.15,16 In both of these investigations, SERSenhancement factors of 105 to 106 are reported. The observedSERS in both of these cases is proposed to be caused by acombination of charge-transfer resonance and a surfaceplasmon resonance. In addition, a resonance in the excitonband gap spectrum has been implicated in the enhancement ofmolecules on MBE-grown CdSe/ZnBeSe nanoparticles.8

In this work, we report on SERS from 4-mercaptopyridine(4-MPy) molecules adsorbed on an etched zinc selenide(ZnSe) surface grown on top of a gallium arsenide (GaAs)substrate using molecular beam epitaxy (MBE). By using theMie theory, we calculate the ZnSe particle size correspondingto the maximum efficiency of near-field enhancement Qnf. Weutilize chemical etching to control the ZnSe particle size toapproximately match that of the calculated value. Using laserexcitation at 514.5 nm, close to the band gap of bulk ZnSe, wefound a large enhancement factor (2 × 106). By analyzing theobserved Raman spectra, we propose that in addition to chargetransfer both band gap and surface plasmon resonances aremost likely contributing factors in the observed Raman signalenhancement.

Received: July 31, 2013Revised: October 6, 2013Published: October 7, 2013

Article

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© 2013 American Chemical Society 23372 dx.doi.org/10.1021/jp407647f | J. Phys. Chem. C 2013, 117, 23372−23377

II. METHODS

Chemicals. 4-Mercaptopyridine (96%) was purchased fromAcros Organics and used without further purification. Acetonewas purchased from Fisher Scientific.Experimental Method. (a). Molecular Beam Epitaxial

Growth of ZnSe Surface on GaAs Substrate. All of thecrystalline layers were grown from elemental solid sources byan MBE system featuring two Riber 2300 growth chambers.One of the chambers is devoted to III−V materials growth,specifically GaAs in this work, and the other is devoted to thegrowth of II−VI materials, specifically ZnSe in this work. Epi-ready (0 0 1) GaAs semi-insulating substrate mounted byindium to a molybdenum block was first introduced in the III−V MBE chamber, where it was heated to remove its nativeoxide layer under arsenic flux. The oxide desorption of thesubstrate was monitored by reflection high-energy electrondiffraction. After the oxide layer was removed, a 100 nm GaAsbuffer layer was grown under As-rich conditions exhibiting astreaky (2 × 4) surface reconstruction. After the GaAs bufferlayer was grown, the GaAs samples were transferred into theII−VI chamber via transfer track modules in an ultrahighvacuum. To improve the III−V to II−VI material interface,exposure to a Zn flux for a period of 30 s, followed by a 50 Ågrowth of ZnSe was performed, all of which was done at 200°C, which is 100 °C lower than the optimal II−VI materialsgrowth. After this low-temperature buffer of ZnSe, the substratetemperature was increased to 300 °C for the remainder of theZnSe growth. The thickness of the ZnSe layer was 1 μm andwas grown under Se-rich growth conditions. The RHEEDpattern exhibited a streaky Se-terminated (2 × 1) surfacereconstruction during the entire ZnSe growth.(b). Atomic Force Micrography. The ZnSe surface was

etched using concentrated sulfuric acid (H2SO4) for 1, 2, and 3min. At the end of the etching period, the surface wasrepeatedly washed for several minutes with water and thendried at room temperature. Atomic force micrograph (AFM)images were acquired using a Nanoscope III (DigitalInstruments) apparatus operated in the air and at roomtemperature. Silicon-sharpened tips with a force constant of0.58 N/m (purchased from Digital Instruments) and aresonance of 260 kHz were used in noncontact mode for theAFM images. At least five images were acquired for each sampleat various locations. Images were recorded in the range of 5 × 5μm2 area and of 1 × 1 μm2 area with a 500 × 500 pixelresolution and a scan rate of 1 μm/s. The dimensions of thenanostructures were measured using Nanoscope 6 imageprocessing software.(c). Raman Spectroscopy. A solution of 4-MPy was

prepared by dissolving powder 4-MPy in pure acetone at 1mM concentration. 4-MPy molecules were adsorbed on theetched ZnSe surface by immersing the clean surface in thesample overnight and then allowing the solvent to evaporate inthe air at room temperature. The surface was then rinsed withacetone for a few minutes to remove the excess molecules. TheRaman spectra of 4-MPy were investigated using Spectra Pro2750 (0.75 m triple grating monochromator 1200 gratings/spectrograph) at the excitation wavelength of 514.5 nmobtained from an Ar+ laser (Spectra Physics). The laser wasfocused on the sample by using a 100× objective lens attachedto a confocal microscope, and the power of the laser on stagewas 0.56 mW. The laser spot size was 2 μm, and the slit widthwas 20 μm. The Raman measurement of the sample was taken

with five accumulations over 10 s of acquisition. To ensure thatthe obtained spectra were comparable, the settings, includinglaser power and exposure time, were all kept constant. Thesilicon line at 520 nm was used to calibrate the observedwavenumbers.

Mie Scattering Calculations. A strong radial dependencewas originally exploited by Mie17 in determining the scatteringefficiencies of metallic spheres. For semiconductor nano-particles, Mie scattering can also take place, and the near-fieldsurface plasmon resonances are strongly size- as well aswavelength-dependent.In this regard, we examine the derivation of Mie scattering

efficiencies due to Messinger et al.18 as well as the MATLABprogram written by Matzler.19 The scattering efficiency (Q) isthe ratio of the power of the scattered light to that of theincident light. A molecule adsorbed on the surface of a spherefeels an electric field in the near-field limit. In the near-fieldlimit, the incident plane wave is distorted to satisfy theboundary conditions of the surface of the sphere. For the near-field scattering efficiency (QNF), it is shown to be

∑= | | + | | + | |

+ + | | | |=

∞

− +Q a n h ka n h ka

n b h ka

2 { [( 1) ( ) ( ) ]

(2 1) ( ) }

nn n n

n n

NF1

21

(2) 21

(2) 2

2 (2) 2(1)

where an and bn are the Mie coefficients and hn(2) are the

Hankel functions of the second kind. The parameter a is theradius of the sphere and k (= 2π/λ) is the wavenumber of theexciting light. The Mie scattering coefficients displayresonances as a function of the parameter x = ka. The onlyinput to the program is the real and imaginary part of therefractive index, which for ZnSe is taken to be 2.98 + 0.4i.20

The excitation wavelength is chosen to be 514.5 nm, and thescattering efficiency may then be plotted as a function ofparticle size.

III. RESULTS

Figure 1 shows (AFM) images of ZnSe surface before and afteretching with acid. AFM images show that before etching (a),ZnSe surface is fairly smooth, whereas protrusions of nanoscalestructures appeared on the surface when it was etched with acidfor 2 min (b,c). Close observation of the surface in Figure 1creveals that the shape of these structures appeared to behemiellipsoidal nanoparticles. Using image-processing software(Nanoscope 6), we determined that the average radius andheight of these particles are approximately 55 and 6 nm,respectively. The AFM images of ZnSe surface etched for 1 minshowed average hemiellipsoidal particles sizes >1 μm. Etchingof the surface beyond 2 min results in considerably diminishedparticle size (in both height and radius) and lack of discernibleRaman signal.The columns in Figure 2 show the measured distribution of

hemiellipsoidal nanoparticles with different diameters on theZnSe surface etched for 2 min. It can be seen from this Figurethat the particle sizes have a broad distribution, while a pluralityof particle sizes are in the range between 110 and 130 nm. Thesolid curve in this Figure represents the dependence of theefficiency of near-field enhancement Qnf on particle size. Thecurve is plotted by fixing the wavelength of incident light at514.5 nm and using a value of refractive index n + iκ = 2.98 +i(0.4). The Figure shows that the optimum particle size is ∼156nm, where the near-field enhancement Qnf reaches its

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maximum. While the majority of the particles on the 2 minetched ZnSe surface are smaller than 156 nm, there isconsiderable overlap between the experimental distributionand the calculated plot, and it can be seen that a reasonablefraction of the particles are within the range of the optimumparticle size, corresponding to the maximum near-field Qnfenhancement. Thus it is likely that at least a fraction of themolecules encounter a plasmon field during this experiment.In Figure 3, we show a comparison between the Raman

spectrum of 4-MPy powder and that of 4-MPy adsorbed on anetched ZnSe surface. In Table 1, we list the wavenumber

measurements of the observed lines in this work as well asthose from previous works.5,6,21−23 It can be seen that thespectrum of 4-MPy on an etched ZnSe surface resembles thatof the powder spectrum, with some shifts in frequency butmore importantly large differences in relative intensity. This issimilar to surface-enhanced Raman spectroscopy (SERS),which is normally carried out on a silver surface. Forcomparison, in Table 1, we list the observed SERS spectrumof 4-MPy on a Ag electrode at −0.8 V (vs SCE) and on PbSnanoparticles. As listed in the Table, the 4-MPy lines observedin this work are almost identical to those that were reported onPbS nanoparticles and on Ag electrode. In particular, it can beseen that the positions of 685, 778, 1281, and 1577 cm−1 linesin ZnSe surface are very similar to those on PbS quantum dots.The most noticeable differences in this Figure are the largeenhancements of the lines at 1025, 1281, and at 1577 cm−1,which are considerably weaker in the powder spectrum. Othernotable enhancements are observed at 685 and at 778 cm−1,which are also quite weak in the powder spectrum. The firstthree lines are the most intense in the spectrum of 4-MPy onthe etched ZnSe surface, while the latter two are at leastcomparable in intensity to the other lines of the spectrum.Referring to the assignments of the bands, it can be seen that

the first intense line at 1025 cm−1 is assigned to a1 symmetry inthe C2v point group, while the lines at 1281 and at 1577 cm−1

are assigned to b2 symmetry. The lines at 685 and at 778 cm−1

are assigned to b1 symmetry. While many of the totallysymmetric a1 modes are seen to be either suppressed orreduced in relative intensity, other nontotally symmetric b1 andb2 modes are enhanced after the adsorption of 4-MPymolecules on the etched ZnSe surface.Figure 4 demonstrates the SERS spectra of 4-mpy molecules

as a function of etching time on the ZnSe surface. Theappearance of totally (a1) and nontotally (b1 and b2) symmetricvibrations can be observed in both spectra resulting from 4-mpymolecules adsorbed on ZnSe surfaces etched for 1 and 2 min.However, this Figure illustrates a remarkable difference in therelative enhancement of the intensity resulting from 4-mpymolecules adsorbed on ZnSe surface. Because the mean particlesize of the 1 min surface is >1 μm, it is far out of range of thesurface plasmon (Figure 2). This gives an indication of the

Figure 1. ZnSe surface (a) before and (b) after etching (5 × 5) μm2

area. (c) ZnSe surface after etching (1 × 1) μm2 area.

Figure 2. Measured distribution of particle diameter of ZnSe on theetched surface. We also show here the dependence of the efficiency ofnear-field enhancement Qnf on the particle size calculated using theMie scattering theory.

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importance of the plasmon resonance to the enhancementfactor.

IV. DISCUSSION

The observed SERS spectra of 4-MPy on etched ZnSe surfacesshow several differences from the normal Raman spectrum andare closer in some ways to the SERS observed on PbS and Agnanoparticles. Among other things, the prominence ofnontotally symmetric (b1 and b2) lines indicates a presence ofcharge-transfer resonance contributions to the overall enhance-ment. Enhancement of both these b1 and b2 modes can also beobserved in PbS quantum dots (Table 1). Charge transfer mostlikely takes place between the VB of ZnSe and the lowestunoccupied molecular orbital (LUMO) levels of 4-MPy. Thelower edge of the conducting band (CB) for ZnSe is −4.09 eV(band gap ∼2.67 eV), whereas the highest occupied molecularorbital (HOMO) for 4-MPy is −9.7 eV (band gap 4.0 to 4.1eV).24−26 The VB of ZnSe lies at −6.76 eV below the vacuum

level, while the LUMO of 4-MPy is ∼1.1 eV above this point,well within the energy needed for a charge-transfer resonance(Figure 5). In addition, ZnSe has a band gap of 2.67 eV, whichis very close to the range of interband transition using theexcitation laser (2.41 eV).Our study also shows that the nontotally symmetric (b1 and

b2) vibrational lines are greatly enhanced on the ZnSe surfaceetched for 2 min, where the particle size has been tailored tosupport a plasmon field. The enhancement of these nontotallysymmetric vibrational lines on the ZnSe surface etched for 1min appears considerably weaker in comparison with that of theZnSe surface etched for 2 min (Figure 4). At 3 min etchingtime, the ZnSe is almost absent from the substrate, and noenhancement is observed.We estimated the enhancement factor from the ZnSe surface

etched for 2 min because the enhancement of 4-mpy molecularlines appears to be the largest from this surface. Theenhancement factor (EF) is the ratio of the Raman intensity

Figure 3. Comparison of the Raman spectrum of 4-mercaptopyridine powder (black) with that of 4-mercaptopyridine molecules adsorbed on aZnSe surface etched for 2 min (red). Excitation is at 514.5 nm.

Table 1. Spectral Lines and Assignments of 4-MPy Observed Bandsa

assignment symmetry Wang, Rothberg 4-MPy solution 4-MPy powder SERS −0.8 V PbS 514.5 nm ZnSe 514.5 nm

1645 16358a a1 1620 1626 1619 1605 1623 16158b b2 1580 1585 1583 1576 1586 1577

1499 149619a a1 1470 1487 1461 1463 1464 145814 b2 1395 1397 13733 b2 1287−1318 1283 1288 1278 1281ip-NH def 1250 1255 1248 1232 12379 (deprotonated) a1 1220 1221 1213 1216 12119 NH+ (protonated) a1 1206 1200 119512 CS a1 1099 1119 1106 1096 1107 112218 a1 1030−1050 1053 1044 1051 1037 10251 a1 1013 1004 989 1009 10114 b1 760 789 780 7786 a1 720 722 722 71311 C−H oop b1 700 655 647 682 685

aColumns 1−3 are the assignments of Wang and Rothberg (ref 21). Column 4 is the solution spectrum of 4-MPy from refs 6 and ref 23. Columns 5and 8 are from this work (Figure 3b). Column 6 is the SERS on Ag electrode (ref 22), and column 7 is the SERS on PbS nanoparticles (ref 5).

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per adsorbed molecule divided by that of the free (or solidphase) molecules. It may be determined by the relationship

=E I N I N/F surf bulk bulk surf (2)

where I and N are the intensities and number of molecules onthe surface and in the bulk powder. We consider the surface ofthe etched ZnSe to be composed of hemiellipsoidalnanostructures with an approximate average radius of 55 nmand an average height of 6 nm. We take for our intensitymeasurement the 1281 cm−1 line of 4-MPy. This line is chosenbecause it is one the most intense lines in the spectrum and iswell separated from its neighbors. After subtracting thebackground intensity, the intensity in the normal Ramanspectrum of this line is ∼564 counts, while from the surface-enhanced 4-MPy spectrum it is about 27 837 counts. In bothspectra, the laser intensity and the focus were kept the same.The intensity ratio (Isurf/Ibulk) is therefore ∼49. Using the probespot size of the laser (2 μm in diameter) and the laserpenetration depth (∼10 μm) and from the density of 4-MPy(1.2 g/cm3), we determine that the number of Nbulk = 2.0× 1011

molecules. Using the formula {π × r × h} to calculate the areaof a hemiellipsoid, we estimate the area of a hemiellipsoidalnanostructure to be ∼1× 10−3 μm2. Using Nanoscope 6 imageprocessing software, we estimated the hemiellipsoidal particledensity on the ZnSe surface to be ∼250 hemiellipsoidalparticles/μm2. Multiplying the particle density by the area ofthe laser spot, we find that there are ∼785 hemiellipsoidalparticles on the etched ZnSe surface that fit within the laser

spot. Multiplying the total number of particles inside the laserspot (785 particles) by its area (1 × 10−3 μm2) gives a totalsurface area of interrogated nanoparticles of 0.785 μm2 by thelaser. Assuming a monolayer coverage on the surface, we findNsurf ≈ 3.9 × 106 by dividing the total surface area ofinterrogation by the cross-sectional area of a 4-MPy molecule.We assume that the vertical cross-sectional area of a 4-MPymolecule is about the same as that of a pyridine molecule,which is 2 × 10−7 μm2.27 This results in an enhancement factorof ∼2 × 106.This value is comparable to most of the early values obtained

for Ag surfaces, implying that semiconductor surfaces may becompetitive with metal surfaces for surface enhancement.Although a comprehensive theory of SERS in semiconductors isnot yet extant, it is unlikely that such a high value can beexplained by a single effect.28 The large enhancement factororiginating from the etched ZnSe surface corresponding to themaximum near-field enhancement (Qnf) is most likely due to acombination of a charge-transfer resonance, a surface plasmonresonance, and a band gap (exciton) resonance, all of which arenear our excitation source. It is impossible in this stage to makequantitative evaluations of the relative contributions of eachsource. Only the molecular resonances can be definitively ruledout because they are far from the exciting source.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: lombardi@sci.ccny.cuny.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Dane Christie for sharing his version of the Miescattering program and help with its implementation. We arealso indebted to Joel De Jesus for technical help concerningmolecular beam epitaxial growth.

This work was partially supported by the Center forExploitation of Nanostructures in Sensor and Energy Systems(CENSES) under NSF Cooperative Agreement AwardNumber 0833180. We are indebted to the National Science

Figure 4. SERS spectra of 4-mercaptopyridine molecules adsorbed on a ZnSe surface etched for 1 min (black) and 2 min (red). Excitation is at 514.5nm.

Figure 5. Energy level diagram of a ZnSe bulk surface and 4-mpymolecules. All levels are measured from the vacuum.

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Foundation (CHE-1041832) for funding of this project. Thisproject was supported by Award No. 2009-DN-BX-K185awarded by the National Institute of Justice, Office of JusticePrograms, U.S. Department of Justice. The opinions, findings,and conclusions or recommendations expressed in thispublication/program/exhibition are those of the author(s)and do not necessarily reflect those of the Department ofJustice.

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