DOI: 10.1007/s00339-006-3812-9

Appl. Phys. A 86, 447–450 (2007)

Materials Science & ProcessingApplied Physics A

b.k. patel∗s. raths.n. sarangis.n. sahu�

HgS nanoparticles:Structure and optical propertiesInstitute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India

Received: 31 January 2006/Accepted: 28 November 2006Published online: 13 January 2007 • © Springer-Verlag 2007

ABSTRACT Nanocrystalline HgS thin films were synthesizedby using an electrochemical route under galvanostatic condi-tions. Quantum size effects have resulted in the change of thesemi-metallic behavior of bulk β-HgS (Eg = −0.5 eV) to semi-conducting behavior with an absorption onset around 1.4 eVas confirmed from optical absorption studies. Glancing angleX-ray diffraction analysis confirms the presence of β-HgS(zinc-blend structure) with prominent crystallographic planes of(200), (220) and (311) in the deposit. This is consistent withresults obtained from transmission electron diffraction studies.The Raman scattering measurements identified a broad 1LOconfined phonon mode at 247 cm−1 which suggests that thecrystalline sizes are small.

PACS 61.10.Nz; 63.22.+n; 81.07.Bc

1 Introduction

Recently there has been considerable interest in thestudy of narrow band gap semiconductors [1, 2, 4–17] dueto their large Bohr exciton radius, aB (i.e. a small effectivemass). When the crystalline size is in the nanometer rangeand is comparable to that of the aB, then the nanocrystal canbe treated as a “particle in a box” which would lead to theobservation of quantum size effects (QSEs) [17]. Such QSEsare responsible for the observation of novel properties suchas band gap widening [1, 4–10]. Due to the small effectivemass [1, 2, 4–9] and comparatively large aB in narrow bandgap semiconductors, the QSE is observed for much largerdimension crystals in comparison to wide band gap semi-conductors which make nanocrystal fabrication easier. Bulkβ-HgS is a narrow band gap semiconductor and behaves likea semi-metal [8] which in general considered as an infraredsensor material like InAs, [9] InSb, [10] PbSnTe, [11] andHgCdTe [12]. Two forms of HgS exist in nature [18] namelyα-HgS (red in color) and β-HgS (black in color). The α-HgShas a band gap, Eg = 2.3 eV and exhibited trigonal structureswith a lattice constant a = 4.149 Å and c = 9.495 Å. Whereas

� Fax: +91 674 2300142, E-mail:sahu@iopb.res.in∗deceased

β-HgS has a band gap Eg = −0.5 eV with a cubic structurehaving a lattice constant a = 5.851 Å. Out of the two phases ofHgS, β-HgS shows interesting properties. Attempts have beenmade to synthesize the β-HgS quantum dots by Schooss etal. [5] using the colloidal chemistry route to prepare samplesin which a shell of HgS (Eg = −0.5 eV) is embedded in a CdS(Eg = 2.5 eV) quantum dot. Furthermore, Mews et al. [2] haveprepared a CdS/HgS heterostructure (i.e. a quantum dot quan-tum well (QDQW) structure) of nanometer dimensions by thewet chemistry route. Although nanostructure HgS films havebeen grown in colloidal and by the wet chemistry route [2, 5],to the best of our knowledge there have been no previousreports on HgS quantum dots grown by an electrochemicalroute. The present work describes the synthesis of β-HgS thinfilms by an electrochemical route and subsequent evaluationof their structural and optical properties using transmissionelectron microscopy (TEM), glancing angle X-ray diffraction(GXRD), Raman scattering and optical absorption measure-ments respectively.

2 Experimental details

Nanocrystalline HgS thin films of different crys-talline sizes have been synthesized on Indium tin oxide (ITO)coated glass and Ti substrates by an electrochemical tech-nique. The precursors used for the synthesis of HgS nanocrys-tals are HgSO4 (0.01 M), Na2S2O3 (0.5 M) and dilute HNO3

acid. The pH of the resultant solution was adjusted to 2 byaddition of HNO3 acid. Different crystalline sizes were ob-tained by controlling the solution temperature, and electro-deposition current density was kept constant at 5.0 mA/cm2.Two samples s1 and s2 were prepared at 300 K and 273 Krespectively for a duration of 5 min each. The structural an-alysis of HgS nanocrystals were carried out by the GXRDstudies using a standard Phillips automatic powder diffrac-tometer with Cu Kα (λ = 1.54 Å) as the source of incidentradiation. TEM was performed using a Phillips CM200 an-alytical TEM fitted with an EDAX analyzer system and op-erated at 200 kV. Raman spectra were recorded at 300 K inbackscattering configuration using a 488 nm line Ar+ laserexcitation and a double monochromator with the standarddetection system. Optical absorption spectra at 300 K wererecorded for HgS nanoparticulate thin films using a JASCOmodel 7800 UV/VIS spectrophotometer for which, a bare

448 Applied Physics A – Materials Science & Processing

FIGURE 1 (a) Glancing angleX-ray diffraction (GXRD) patterns ofelectro-deposited β-HgS on ITO (s1)and ITO substrate samples. (b) Brightfield (BF) image of the sample, s1shows two dimensional patterns. Theblack contrast are deposited mer-cury sulfide. (c) Corresponding con-vergent beam electron diffraction(CBED) patterns of the sample (s1),which indicates that the particles arein a crystalline phase. The insets of(b) and (c) correspond to the selectedarea diffraction (SAD) pattern of thesample s1 showing a crystalline cubicphase

ITO glass plate was placed in front of the reference beamwhile the ITO glass with HgS film was placed in front of thesample beam.

3 Results and discussion

Figure 1 shows the GXRD pattern (s1) of HgS de-posit along with the contribution from the ITO substrate. Theprominent crystallographic planes (200), (220) and (311) cor-responding to the β-HgS have been identified. Note the fullwidth of half maxima (FWHM) of GXRD peaks gets broad-end as the crystalline size become smaller (not shown inFig. 1). The average crystalline size of the HgS sample hasbeen estimated from the (220) peak using the Scherrer equa-tion [16] and is found to be 15 nm. The morphology of HgSnanocrystals was investigated by TEM as shown in Fig. 1b.The individual size of the HgS nanoparticles were difficult tomeasure from the bright field (BF) images, however, it doesshow a two-dimensional crystalline growth. A convergentbeam electron diffraction (CBED) pattern of a nano-phaseHgS (s1) is shown in Fig. 1c. The diffraction disks lie withincircular and annular bands known as Laue zones. The centralzone is the zeroth order Laue zone and the rest of the annu-lar bands are collectively called higher-order Laue zones. ThisCBED patterns clearly indicates that the deposited HgS is in

crystalline phase. The selected area diffraction (SAD) patternof s1 as shown in the inset of Fig. 1b and c clearly showsthe formation of crystalline particles. The diffraction planesanalysed from the SAD patterns are in excellent agreementwith that obtained from GXRD measurements and confirmthe zincblende structure in the deposit.

The Raman active phonon modes of cubic HgS (s1)were investigated by Raman scattering measurements using a488 nm Ar+ laser at room temperature (300 K) and the spectraof the two different regions of the same sample (s1) is shownin Fig. 2a. Note from Fig. 2a that the two 1LO optic phononmodes of β-HgS corresponding to the two regions are found tobe shifted to 245.3 cm−1 and 246.9 cm−1 from the bulk 1LOphonon mode of 253 cm−1 [14]. Two different 1LO phononmodes of the same samples from two different regions sug-gests the presence of a size distribution in the deposit. Theshift of the 1LO Raman mode at lower frequency from its bulkvalue suggests phonon confinement for which the crystallitesizes become comparable to Bohr’s exciton radius. In thatcase, the q = 0 selection rule relaxes for 1LO phonon modesand Raman contribution to areas other than the zone center,contribute to the Raman scattering. It is expected that all thephonons with wave vectors in the range 0 ≤ q < h

D (shownschematically in Fig. 2b) will participate in the Raman scatter-ing process where D is the average diameter of spherical HgS

PATEL et al. HgS nanoparticles: Structure and optical properties 449

FIGURE 2 (a) Room temperature(300 K) Raman spectra for samples1. Spectra-1 and -2 are from twodifferent regions of the same sam-ple showing the size distribution.(b) Schematic illustration of three di-mensional phonon confinement. Theshift of the phonon peak is δω due tosize effects

crystallites. In the present case, HgS-LO mode is observed ata frequency around 246.9 cm−1 which is red shifted around∼ 7.0 cm−1 relative to its bulk LO mode. Similar observa-tion of red shift in the case of nano-phase HgS in a QDQWstructure is reported by Mews et al. [2].

Figure 3 shows the Tauc plots, (αE)2 vs. E, for the sampless1 and s2 respectively. The electron transition at the fundamen-tal absorption edge is found to obey the direct transition lawsimilar to the nano-phase PbS sample [6]. The band gap en-ergies found from the extrapolation of the linear part of theplot to zero absorption were significantly higher than the cor-responding bulk β-HgS. The measured band gaps are 1.38 eVfor s1 and 2.06 eV for s2 respectively. Similar band gap tun-ing in the visible and infrared region has also been reportedby Rath et al. [3] and Rogach et al. [4] respectively for nano-phase HgTe. In general, the Hg-compound semiconductorsviz. HgTe, HgSe, HgS etc. belonging to the II–VI group showa zero or negative fundamental gap with inverted band struc-tures in the bulk form [13]. The recent report on HgS in the

FIGURE 3 (a) Optical absorption spectra (αhν)2 vs. hν plot of electro-deposited HgS samples. Band gap is found to be less for the sample preparedfor room temperature. (b) Band structure along the symmetry line connect-ing X- and L-point of Brillouin zone. Optical transition occurs at Γ-pointbetween Γ6 and Γ8 symmetry lines. Confined gap of the nano-phase β-HgSis Eg = E(Γ6) ∼ E(Γ8) = 1.38 eV for s1 and 2.06 eV for s2 respectively

QDQW structure of Schooss et al. [5] suggests the 1s–1stransition in the optical absorption studies. However, in thepresent case, being a quantum dot (QD), HgS we also couldsee a blue shift in their optical absorption in the near infrared(NIR) region which needs explanation.

Our GXRD and TEM studies reveal the β-HgS to be crys-tallised in the zincblende structure and in cubic phase. Theimportant symmetry points in terms of group theory notationcan be given as the Γ -point at (000), and the X-point at (001)and the L-point at

( 12

12

12

). For semiconductor nanostructures

showing QSE, the X and L points are less important whereasthe Γ -point has significance as the band discretization domi-nates at the Γ -point and the band gap is thus widened. Hence,the optical transition is now dominated at the Γ -points by Γ8and Γ6. This corresponds to the highest occupied molecularorbital (HOMO) and the lowest unoccupied molecular orbital(LUMO) transition of nano-phase HgS. Therefore, the Γ8–Γ6

transition energies are 1.38 and 2.02 eV for samples s1 and s2respectively and are blue shifted from their bulk value simi-lar to the 1s–1s transition in the QDQW structure [5]. Further,the optical absorption spectra for samples s1 and s2 show fea-tureless absorption in contrast to Schooss et al. [5] indicatingthe absence of excitonic absorption. Absence of excitonic ab-sorption in our case may be attributed to two main reasons: (i)the weak excitonic energy due to strong Coulomb screeningin narrow band gap semiconductors and (ii) the existing sizedistribution [6].

4 Conclusion

Electrochemical synthesis of nano-phase HgS hasbeen reported for the first time. Structural studies of nano HgSshow the cubic zincblende structure as revealed from GXRDand TEM studies. QSE has been observed from optical ab-sorption studies. A band gap in the range of 1.4 eV to 2.0 eVhave been obtained for this material which suggests band gapwidening in the narrow band gap semiconductor due to SQE.The 1LO phonon mode is found to be shifted by � 7 cm−1

450 Applied Physics A – Materials Science & Processing

from its bulk value which again suggests that the HgS sizes aresmall.

ACKNOWLEDGEMENTS Thanks are due to Prof. S.N. Be-hera and Dr. R.K. Choudhury for their helpful discussions and constantencouragement.

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