Stibnite inverse opal

Micdoi:

www.ietdl.org

Published in Micro & Nano LettersReceived on 24th September 2007Revised on 6th December 2007doi: 10.1049/mnl:20070059

ISSN 1750-0443

Stibnite inverse opalA.Z. Khokhar1 R.M. De La Rue1 B.M. Treble1 D.W. McComb2

N.P. Johnson11Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, UK2Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UKE-mail: [email protected]

Abstract: A 3D photonic crystal (PhC), synthetic opal, has been constructed by self-assembly of sub-micrometresilica spheres and designed for operation in the visible spectral range. Because of its low refractive index contrast(RIC), this PhC does not exhibit a full photonic band gap (PBG), but this property could be achieved by increasingsufficiently the RIC. It is not easy to find a high refractive index material that is transparent in the visible spectralrange, but we report here on a method for producing 3D PhC structures with increased RIC, using incorporationof stibnite (Sb2S3) into a silica opal PhC. The template is infiltrated with the precursor (Sb[CS(NH2)2]3Cl3), withsubsequent thermal decomposition at 6008C to form Sb2S3 in the voids of the opal PhC. Formation of Sb2S3and removal of silica spheres by a chemical etching process can produce a structure that exhibits a full PBG inthe visible spectral range. The optical properties of the inverse Sb2S3 opal have been measured and comparedwith theoretical calculations.

1 IntroductionPhotonic crystals (PhCs) have been described assemiconductors for light, in which photons are used insteadof electrons [1]. When PhCs are periodic in all three spacedimensions, they are routinely called 3D PhCs. The 3DPhC is interesting because it can act as a complete ‘cage’for photons, when the property of a full photonic band gap(PBG) is exhibited. In the last decade, many successfulattempts have been made to fabricate the 3D PhC eitherby top-down [2, 3] or bottom-up [4] techniques.Potentially, the bottom-up approach is easier and lessexpensive, but it typically involves more reliance on thetechniques of chemistry. By using chemical techniques, 3DPhC can be realised using silica (SiO2) [5] or polymer (e.g.polystyrene) spheres [6]. Such 3D PhCs are known as asynthetic opal and exhibit a face-centred cubic (FCC)structure.

Possibilities have been identified for short-range opticalcommunications systems that use wavelengths in the visibleor very near infrared (NIR) parts of the spectrum andtherefore there is a need to fabricate PhC structures withstop bands in this range, including structures that exhibit

ro & Nano Letters, 2008, Vol. 3, No. 1, pp. 1–610.1049/mnl:20070059

full PBG properties. Once an inverse opal with full PBGbehaviour has been demonstrated, it becomes possible toconsider the use of light emitting polymers, dyes ornanodots [7] embedded in such full PBG structures.

Synthetic opal, whether made from silica spheres orpolystyrene spheres, does not exhibit full PBG behaviour[8, 9]. Such opal structures do have a partial stop band forpropagation in the ,1 1 1. direction, between bands 2and 3, because of the low refractive index contrast (RIC)between the sphere material and the background, that is,air. Sozuer et al. [10] have shown that, if the RIC . 2.89,full PBG behaviour can be obtained in such FCCstructures, between bands 8 and 9. The RIC of the opalcan be increased in magnitude (but with a change in sign)by infiltration of high refractive index material and can befurther increased by removing the silica or polymer spheresafter infiltration. A full PBG has been achieved in the nearinfrared by infiltration of silicon [11] and germanium [12]into the opal voids and subsequent removal of the spheresafter infiltration.

Obtaining a full PBG in the visible part of the spectrumrequires both a sufficiently small lattice constant (i.e. sphere

1

& The Institution of Engineering and Technology 2008

2

& T

www.ietdl.org

sizes on the order of 100–200 nm in the opal template) and ahigh enough refractive index but transparent material. Oneof the small number of candidate materials is stibnite(Sb2S3). The refractive index of crystalline Sb2S3 variesbetween 3.69 and 3.79 [13] and its absorption edgeoccurs at around 690 nm [14, 15], at the upper limit ofthe visible wavelength range. It is possible to infiltrateSb2S3 into the voids of opal and thereby to increase themagnitude of the RIC and for a full PBG to be achievableby removing the spheres. Previous work by Juarez et al.[16, 17] has described the infiltration of Sb2S3 into opalvoids to obtain a full PBG in the very NIR regime (700–800 nm). In their work, they infiltrated the Sb2S3 into theopal voids by using a chemical bath deposition andhydrolysis method.

Here, we report a simple and inexpensive technique forinfiltrating the crystalline Sb2S3 into the voids of the silicaopal. Infiltration was carried out during the thermaldecomposition of the precursor (Sb[CS(NH2)2]3Cl3) athigh temperature. To increase the RIC, the spheres wereremoved by wet etching after the formation of the Sb2S3.Along with the full PBG in the visible to NIR, two higherwavelength stop bands are also found in the NIR regime.The optical properties are also compared with a computedband-structure diagram for the Sb2S3 inverse opal.

2 Synthesis of silica spheresThe silica spheres for the opal were produced using theStober process [5]. The reagents used were tetraethylorthosilicate (TEOS) (Aldrich 99.999%), ammoniumhydroxide (Aldrich), ethanol (Fisher 99.99%) and de-ionised (DI) (18 MV) water. Ethanol was distilled prior touse in order to remove traces of water and other impurities.The mono-dispersed silica spheres were produced by thehydrolysis and condensation of TEOS, using ammoniumhydroxide as a catalyst. The reaction mixture was agitatedusing a magnetic stirrer at 268C for 24 h. After thecompletion of the reaction, the spheres were removed fromthe solution by using vacuum filtration and then dried outand stored in a vial. Silica spheres with mean diameters inthe range 207–543 nm were synthesised by using thismethod. The variation of the mean diameter of the silicaspheres was achieved by varying the molar concentration ofthe ammonia (0.5–2 M) and water (1–16 M), whereas theconcentration of the TEOS (0.3 M) remained fixed.Transmission electron microscopy was used for the sizemeasurements, with at least 200 individual spheres beingmeasured and the standard deviation found to be in therange 2–5%.

3 Fabrication of silica opalSilica opal was formed on a silicon substrate by using thecapillary growth of 406 nm diameter silica spheres. The406 nm sphere size was selected in order to avoidoverlapping the electronic band gap of the Sb2S3 with the

he Institution of Engineering and Technology 2008

first photonic stop band of the PhC formed by infillingSb2S3 into the silica opal. The substrate was cleaned withacetone, methanol and DI water and blown dry withnitrogen. About 0.0350 gm of silica spheres was dispersedin 5 ml of ethanol and the silicon substrate was placed atan angle of 658 in a small vial, which was kept in atemperature-controlled environment [18]. The temperaturewas set at 468C and the relative humidity was varied from5% to 10%. After 36 h, the samples were found to be driedout and a �12 mm thick opal film of 406 nm silica sphereswas produced.

4 Synthesis of Sb2S3The synthesis of crystalline Sb2S3 involves two steps. Thefirst step is the preparation of the precursor, that is,Sb[CS(NH2)2]3Cl3. The precursor was prepared fromantimony (III) chloride (SbCl3) and thiourea (CS(NH2)2)in solution in methanol. The second step is the thermaldecomposition of the precursor to form the Sb2S3

SbCl3 þ 3CS(NH2)2 ! Sb[CS(NH2)2]3Cl3

2Sb[CS(NH2)2]3Cl3 ! Sb2S3 þ 3S[C(NH2)2]2 þ 3Cl2

X-ray diffraction (XRD) was carried out on the materialproduced, after raising it to various temperatures in therange 200–4008C as shown in Fig. 1. The XRD indicatesthat the Sb2S3 starts to form by thermal decomposition ofthe precursor at 2008C. At 4008C, pure Sb2S3 appears tobe formed, since small peaks not associated with Sb2S3disappear, as shown in the shaded area of Fig. 1. All otherprominent peaks are associated with Sb2S3. The Sb2S3XRD spectrum formed at 4008C is in close agreement withthe reference spectrum for Sb2S3 (Fig. 2).

Figure 1 XRD pattern of Sb2S3a 2008Cb 3008Cc 4008CThe shaded area indicates that the small peaks not associatedwith Sb2S3 disappear as the temperature to which the materialhas been elevated increases

Micro & Nano Letters, 2008, Vol. 3, No. 1, pp. 1–6doi: 10.1049/mnl:20070059

Micdoi:

www.ietdl.org

Figure 2 Reference spectrum for Sb2S3 (grey lines) [21]: spectrum in black is for Sb2S3 formed during thermal decompositionof the precursor at 4008C

5 Infiltration of Sb2S3Sb2S3 is formed during the thermal decomposition of theprecursor at high temperatures and in the pure form beginsto melt at 5508C [19]. In a single step, a thin layer ofprecursor material was placed on (1 1 1)-oriented silica opalgrown on a silicon substrate and a sandwich structure wasrealised using a silicon substrate on top of the precursor.The whole assembly was heated to form Sb2S3 that wassubsequently melted and infiltrated into the opal voids bycapillary action. The following conditions were used: the

Figure 3 SEM image of 3D PhC made of 406 nm SiO2

spheres, after infiltration of Sb2S3 at 6008C for 5 h,followed by 7508C for 4 h to increase the mechanicalstrength of the sample

The inset shows SiO2 opal before infiltrationThickness of domains is �12 mm


sample was held in a furnace with flowing nitrogen(5 sccm) and the temperature was set at 6008C for 5 h,after ramping up from ambient at 208C/min. After the 5 hat 6008C, the temperature was raised to 7508C for afurther 4 h, a process which was found to increase themechanical stability of the composite. The sample wascooled by decreasing the temperature at a rate of �28C/min in a flowing nitrogen atmosphere. After the removal ofthe top silicon substrate, the sample was examined in ascanning electron microscope (SEM). Excellent infiltrationof the polycrystalline Sb2S3 into the opal voids wasobserved, as shown in Fig. 3.

Figure 4 SEM image of the inverted Sb2S3 sample after wetetching by using diluted (100:1) HF

The inset shows small area covered with Sb2S3 inverse opal afterwet etching by using concentrated (5:1) HF

3


4

&

www.ietdl.org

6 Sb2S3 inverse opalIn order to increase the magnitude of the RIC, the SiO2

spheres were removed by using wet etching. For thispurpose, hydrofluoric acid (HF) was used. The area of thethin inverse film was increased by changing the HFconcentration. Highly concentrated HF (5:1 withammonium fluoride) was found to dissolve the silicaspheres within 1 min and produce a small area(�25 � 25 mm) on the sample covered with the inverseSb2S3 thin film. However, diluted HF (100:1 with water)produces larger areas, �1 mm2, covered with the inverseSb2S3 thin film and with the silica spheres removed, after�30 min. After HF treatment, the sample was rinsed withDI water to remove the traces of HF and blown dry withnitrogen. The inverse sample was examined under theSEM, as shown in Figs. 4 and 5.

7 Measurements of the opticalpropertiesWe have investigated the optical reflectance properties of theSiO2 opal loaded with the Sb2S3 and have also investigatedthe Sb2S3 inverse structure produced by the removal of theSiO2 spheres. Normal incidence reflectance measurementswere carried out using a monochromated white light sourcecoupled to an optical microscope with �5 objective. Thereflected light was detected by using a silicon detectorfollowed by a lock-in amplifier. Figs. 6 and 7 present thebaseline-corrected reflectance measurements before andafter the removal of SiO2 spheres, respectively, whereas theinsets show reflection measurements before baselinecorrection. The SiO2 opal loaded with Sb2S3 does not havea sufficiently large RIC to open a full PBG. For this case,three peaks are clearly observable along the G–L directionin the reflectance measurements, as shown in Fig. 6.Removal of the SiO2 spheres is necessary to increase themagnitude of the RIC. For Sb2S3, an RIC . 3.5 gives the

Figure 5 SEM image of the Sb2S3 inverted sample after wetetching at high magnification

The infiltration of Sb2S3 appears to be almost total

The Institution of Engineering and Technology 2008

possibility of a full PBG in the visible spectrum atwavelengths �702 nm. In the reflectance measurementshown in Fig. 7, the shaded area indicates the position ofthe full PBG. This spectrum also shows two shoulders athigher wavelengths denoted at points a and b (inset ofFig. 7). These shoulders are due to stop bands at higherwavelengths, which may be resolved by using a Gaussian

Figure 6 Baseline-corrected reflectance measurement ofopal made of 406 nm SiO2 opal loaded with Sb2S3, madeusing a �5 microscope objective

The inset shows a reflectance measurement before baselinecorrection.

Figure 7 Baseline-corrected reflectance measurement ofinverted Sb2S3 sample after wet etching

Reflectance measurement is made by using a �5 microscopeobjectiveThe spectrum represents the full PBG at 702 nm indicated by theshaded areaThe inset shows reflectance measurements before baselinecorrectionThe arrows indicate the two stop bands at the longer wavelengthin the spectrumTwo fitted Gaussian functions were used to resolve the higherwavelength stop bands at positions denoted by ‘a’ and ‘b’ in thespectrum


Micdoi:

www.ietdl.org

deconvolution function (Syner JY software). We havecompared these two stop bands in Fig. 7 with the banddiagram shown in Fig. 8. These stop bands have theirpeaks at 820 (between bands 5 and 6) and 940 nm(between bands 2 and 3) and are also shown in Fig. 7.Such higher wavelength stop bands with a full PBG werealso reported by Juarez et al. [16, 17].

The optical properties were compared with the theoreticalcalculations based on a band diagram of the FCC opal. Theplane wave expansion method was used to plot the banddiagram [20]. It was assumed that the filling factor of theSb2S3 in the opal voids is 100%. Fig. 8 shows the first 18bands of the FCC opal with the RIC . 3.5. The cross-hatched region in Fig. 8 represents the full PBG openingin the visible regime between bands 8 and 9. The shadedarea represents the stop band for the ,1 1 1. direction ofthe FCC opal. The inset in Fig. 8 shows the band diagramof silica opal infilled with Sb2S3. There is no completeband gap between the bands 8 and 9, as the RIC is �2.59.

8 Discussion and conclusionsSb2S3 is one of the high refractive index materials that havean electronic band gap in the visible part of the spectrum,albeit at the top end of the visible wavelength range. Wehave adopted a simple capillary method to infiltrate theSb2S3 into the voids of synthetic opal. The infiltration wascarried out by melting the Sb2S3 at 6008C for at least 5 h.The refractive index of the crystalline Sb2S3 is .3.5.Infiltration of Sb2S3 into the silica opal voids does not givea sufficient RIC to open up a full PBG. However, removal

Figure 8 Band diagram for synthetic opal made from airspheres with dielectric constant 1 in inverse Sb2S3 opalwith refractive index 3.79

X, U, L, G, W and K are the symmetric points of the Brillouin zonesThe crosshatched area represents the full PBG between bands 8and 9The shaded area represents the G–L, that is,,1 1 1. direction ofthe FCC opalThe inset shows the band diagram for synthetic opal made fromsilica spheres with refractive index 1.46 infilled with Sb2S3


of the spheres after infiltration by using dilute HF increasesthe RIC to above 3.5 and opens up the possibility of a fullPBG in the visible, centred at 702 nm. Two stop bands athigher wavelengths were also found, along with the fullPBG for the fundamental stop band. Control of sphere sizeis important in producing a full PBG at wavelengths,where the absorption of the Sb2S3 is acceptably small.Reflectance measurements were compared with the banddiagram and found to be in good agreement. In the presentwork, we have carried out reflectance measurements atnormal incidence. More comprehensive experimentalmeasurements, for example angle-resolved measurements,are needed to verify that a full PBG has been produced. Insuch a case, the full PBG peak region should not bestrongly dependent on the angle of incidence of the light.Such measurements will be the subject of future work.

9 AcknowledgmentsThe authors thank Mr. Andriy Dyogtyev for calculating theband structure of the Sb2S3 inverse opal. We alsoacknowledge support from the Metamorphose Network ofExcellence.

10 References

[1] JOHN S., TOADER O., BUSCH K.: ‘Photonic band gap materials:a semiconductor for light’ in ‘Encyclopedia of PhysicalScience and Technology’ (Academic Press, 2001, vol. 12),pp. 1–23

[2] SUBRAMANIA G., LIN S.Y.: ‘Fabrication of three-dimensionalphotonic crystal with alignment based on electron beamlithography’, Appl. Phys. Lett., 2004, 85, (21),pp. 5037–5039

[3] CAMPBELL M., SHARP D., HARRISON M.T., ET AL.: ‘Fabrication ofphotonic crystals for the visible spectrum by holographiclithography’, Nature, 2003, 404, pp. 53–56

[4] BLAADEREN A.V.: ‘Opals in a new light’, Science, 1998, 282,pp. 887–888

[5] STOBER W., FINK A., BOHN E.: ‘Controlled growth ofmonodisperse silica spheres in the micro size range’,J. Coll. Interface Sci., 1968, 26, pp. 62–69

[6] GOODALL A.R., WILKINSON M.C., HEARN J.: ‘Mechanism ofemulsion polymerization of styrene in soap-free system’,J. Polym. Sci., 1977, 15, pp. 2193–2218

[7] KHOKHAR A.Z., DE LA RUE R.M., JOHNSON N.P.: ‘Modifiedemission of semiconductor nano-dots in 3-D photoniccrystals’, IET Circuits Devices Syst., 2007, 1, (3), pp. 210–214

5


6

& T

www.ietdl.org

[8] REYNOLDS A., LOPEZ-TEJEIRA F., CASSAGNE D., ET AL.: ‘Spectralproperties of opal-based photonic crystals having a SiO2

matrix’, Phys. Rev. B, 1999, 60, pp. 11422–11426

[9] PAVARINI E., ANDREANI L.C., SOCI C., ET AL.: ‘Band structureand optical properties of opal photonic crystals’, Phys.Rev. B, 2005, 72, p. 045102(1–9)

[10] SOZUER H.S., HAUS J.W., INGUVA R.: ‘Photonic bands:convergence problems with the plane-wave method’,Phys. Rev. B, 1992, 45, (24), pp. 13962–13972

[11] BLANCO A., CHOMSKI E., GRABTCHAK S., ET AL.: ‘Large-scalesynthesis of a silicon photonic crystal with a completethree-dimensional bandgap near 1.5 micrometres’,Nature, 2000, 405, pp. 437–440

[12] MIGUEZ H., CHOMSKI E., GARCIA-SANTAMARIA F., ET AL.: ‘Photonicbandgap engineering in germanium inverse opals by chemicalvapor deposition’, Adv. Mater., 2001, 13, (21), pp. 1634–1637

[13] GHOSH C., VARMA B.P.: ‘Optical properties of amorphous andcrystalline Sb2S3 thin films’,Thin Solid Films, 1979,60, (1), pp. 61–65

[14] VEDESHWAR A.G.: ‘Optical properties of amorphous andpolycrystalline stibnite (Sb–S3) films’, J. Phys. III, 1995, 5,pp. 1161–1172

he Institution of Engineering and Technology 2008

[15] NAIR M.T.S., PENA Y., CAMPOS J.: ‘Chemically deposited Sb2S3and Sb2S3–CuS thin films’, J. Electrochem. Soc., 1998, 145,(6), pp. 2113–2120

[16] JUAREZ B.H., RUBIO S., SANCHEZ-DEHESA J., ET AL.: ‘Antimonytrisulfide inverted opals: growth, characterization, andphotonic properties’, Adv. Mater, 2002, 14, (20),pp. 1486–1490

[17] JUAREZ B.H., IBISATE M., PALACIOS J.M., ET AL.: ‘High-energy photonic bandgap in Sb2S3 inverse opals bysulfidation processing’, Adv. Mater, 2003, 15, (4),pp. 319–323

[18] MCLACHLAN M.A., JOHNSON N.P., DE LA RUE R.M., ET AL.: ‘Thinfilm photonic crystals: synthesis and characterisation’,J. Mater. Chem, 2004, 14, pp. 144–150

[19] WEAST R.C., ASTLE M.J.: ‘CRC handbook of chemistry andphysics’ (CRC Press, 1979)

[20] HO K.M., CHAN C.T., SOUKOULIS C.M.: ‘Existence of a photonicgap in periodic dielectric structure’, Phys. Rev. Lett., 1990,65, (25), pp. 3152–3155

[21] Department of Geographical and Earth Sciences,University of Glasgow, UK


Documents

Stibnite inverse opal