Fabrication and characterization of topological insulator Bi2Se3 nanocrystalsS. Y. F. Zhao,1 C. Beekman,1 L. J. Sandilands,1 J. E. J. Bashucky,1 D. Kwok,2 N. Lee,2

A. D. LaForge,3 S. W. Cheong,2 and K. S. Burch1,a�

1Department of Physics and Institute of Optical Sciences, University of Toronto, 60 St. George Street,Toronto ON M5S 1A7, USA2Department of Physics and Astronomy, Rutgers Center for Emergent Materials, Rutgers University,136 Frelinghuysen Road, Piscataway, New Jersey 08854, USA3Department of Physics, University of California, Santa Cruz, 1156 High Street, Santa Cruz,California 95064, USA

�Received 14 December 2010; accepted 3 March 2011; published online 6 April 2011�

In the recently discovered class of materials known as topological insulators, the presence of strongspin-orbit coupling causes certain topological invariants in the bulk to differ from their values invacuum. The sudden change in invariants at the interface results in metallic, time reversal invariantsurface states whose properties are useful for applications in spintronics and quantum computation.However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices andprobing the surface. To this end we have produced 2 nm thick nanocrystals of the topologicalinsulator Bi2Se3 via mechanical exfoliation. For crystals thinner than 10 nm we observe theemergence of an additional mode in the Raman spectrum. The emergent mode intensity togetherwith the other results presented here provide a recipe for production and thickness characterizationof Bi2Se3 nanocrystals. © 2011 American Institute of Physics. �doi:10.1063/1.3573868�

Topological metallic surface states are predicted to havenumerous properties that are useful for spintronics and quan-tum computation.1,2 A challenging aspect of this research hasbeen to isolate surface state contributions to the measuredproperties of topological insulators.3–7 Studying nanometerthick crystals allows one to tune the chemical potential withelectric field,8–12 as well as observe modifications of the ex-citation spectrum produced by interactions of top and bottomsurfaces.11,13–16 One option for producing thin crystals ismechanical exfoliation. This method, together with Ramanspectroscopy, has proven to be extremely fruitful in the studyof graphene17,18 and other nanocrystals.19,20 Indeed, Ramanprovides direct access to the phonon spectrum and can beused to map the thickness17,21 or doping level22 over a largearea.

Similar studies of the topological insulators Bi2Se3 andBi2Te3 have been attempted. To date, these experiments havebeen limited to exfoliated Bi2Se3 crystals �10 nmthick12,23,24 or Bi2Te3 where the bulk gap is small.8,9 A lim-iting factor in these experiments was the strong optical ab-sorption of these compounds, making the identification ofthin crystals difficult on Si /SiO2 substrates. With this inmind, we have mechanically exfoliated Bi2Se3 crystals onMica, enabling us to optically identify crystals only 2 nmthick. By systematically studying these crystals with Ramanspectroscopy and optical and atomic force microscopies�AFM� we have devised a method for characterizing thethickness of Bi2Se3 nanocrystals using noninvasive, all-optical methods. Specifically, an additional mode appears inthe Raman spectra for ultrathin ��10 nm� crystals. The ob-served thickness dependence of the emergent mode intensitycan be used for thickness verification of nanocrystals viaRaman measurements.

Bi2Se3 forms a rhombohedral lattice in which the unitcell is composed of three five-layer stacks known as quin-tuple layers �QLs�. A unit cell measures 2.87 nm along thec axis, and 1 QL is �0.96 nm thick.25 Atoms are arrangedinto planar hexagonal sheets with the sequence–�Se�2�–Bi–Se�1�–Bi–Se�2��–.26 The superscripts indicatethe structural in-equivalence of the Se ions, with the Se�1�

atom at a center of inversion within a QL. Therefore, oneexpects phonon modes to be exclusively infrared �IR� or Ra-man active. The weak van der Waals bonds between neigh-boring Se�2� planes enables mechanical exfoliation. UltrathinBi2Se3 nanocrystals could be identified after exfoliation byoptical microscopy in transmission mode. The crystal thick-ness was subsequently determined using AFM.26

Raman spectroscopy ��=532 nm, spot size �1 �m�confirmed that these nanocrystals are indeed Bi2Se3 and wasused to study the evolution of the crystal lattice structurewith thickness. A raman spectrum �I���� for a bulk crystal isshown in Fig. 1�a�, where we observe a strong phonon modeat 175 cm−1 and the onset of a mode below 150 cm−1. Pre-vious results25 suggest the higher energy mode correspondsto a Raman active A1g mode and the latter to an Eg mode, inaccord with group theory predictions for phonons at the Bril-louin zone center �q=0� probed by optical experiments. In-deed, the five atoms in the unit cell should lead to 12 opticalmodes, each of which is either exclusively Raman or IRactive.25 We also observe a broad shoulder between 200 and350 cm−1, similar to a feature observed in a recent IR studyof bulk Bi2Se3 �see Fig. 2�b��.27 The presence of this featurein both Raman as well as IR and its broad lineshape suggestthat it is due to two-phonon excitations.

To quantitatively analyze the evolution of the spectra, wefit the measured Raman data with multiple Lorentzian oscil-lators in the form: I���= I0+�i�Ai�i / �4��−Ei�2+�i

2�� wherei ranges from 1 to 3 or 4, depending on thickness, I0 accountsfor the background, Ei is the center, �i is the width, and Ai isthe area of peak i. The resulting fit for the bulk spectruma�Electronic mail: kburch@physics.utoronto.ca.

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with three oscillators is shown in Fig. 1�a�, where the centerfrequency of the Eg mode �peak 1� was fixed at 131.5 cm−1

based on previous studies.25 Turning to Fig. 1�b� we examinethe evolution of the Raman spectra with varying crystalthicknesses, where an enhancement of the overall signal isobserved with decreasing number of QL. As is discussed

below this enhancement results from multiple reflections inthe Bi2Se3 crystal. Perhaps more surprisingly, an additionalmode emerges at 158 cm−1 as the crystal thickness is re-duced �see Fig. 1�c��. Indeed, the use of only three oscilla-tors, which worked well for crystals thicker than 10 QL �Fig.1�a��, results in a large difference around 158 cm−1 betweenthe fit �dashed dotted line� and the spectrum. By simply add-ing another mode �i=4�, the fit �solid line� agrees very wellwith the data. The emergence of the mode at 158 cm−1 forultrathin crystals suggests it can be used to verify samplethickness.

We explore this possibility in Fig. 2, where we plot theratio of the emergent mode intensity with the main peakintensity �A4 /A2�. For thicknesses below 10 QL this ratioincreases and agrees well with � / �QL−2� behavior �dashedline�, which corresponds to the relative weight of surface andbulk modes. Indeed, the intensity of a particular mode isproportional to the volume over which the light can emanate.For a surface mode, this volume is independent of thickness.In contrast, for a bulk mode the volume probed is propor-tional to the crystal thickness �for thicknesses less than thepenetration depth�. Unfortunately, due to the detection limitof our instrument, we are not able to convincingly detect themode for A4 /A2�0.02.

While the mode appears to have its origin in the surface,it could also be due to Fabry–Perot �FP� interference in theMica substrate. To rule this out, we repeatedly performedRaman spectroscopy on a single 3 QL nanocrystal and sub-sequently cleaved the back surface of the Mica to reduce itsthickness. The resultant spectra all overlapped �not shown�,implying that FP interference in the Mica substrate can beneglected. Nonetheless, for a fixed substrate thickness, theinterference effects due to multiple reflections in the Bi2Se3nanocrystals will change as the crystals are thinned. To checkthe FP effects on the Raman spectra we have performed acalculation similar to the one shown to work well ingraphene and Bi2Sr2CaCu2O8 nanocrystals.20,21 In Fig. 3, weplot the measured and the calculated intensity �FP model� asa function of crystal thickness for the main mode�175 cm−1�. Changes in the ratio �A4 /A2� due to FP effectsare plotted in Fig. 2�a� and only reveals a very small depen-dency on crystal thickness, which is opposite to what we

FIG. 1. �Color online� Raman spectra for Bi2Se3. �a� Spectrum for the bulkcrystal �linesymbols� with the corresponding fit �solid line� consisting ofthree Lorentzian oscillators �dotted lines�. �b� Spectra for crystals of varyingthicknesses, 3→16 QL and bulk. �c� Spectrum for 3QL nanocrystal. Thedata �linesymbols� is fit with four Lorentzian oscillators �solid line�. Thefit resulting from three oscillators is shown for comparison �dashed dottedline�.

FIG. 2. �Color online� �a� Ratio of emergent peak intensity to the main peakintensity �determined from fits� as function of crystal thickness. The dashedline is a guide to the eye and plotted to show the �� / �QL−2�� trend of theratio. The gray area for A4 /A2�0.02 indicates the detection limit of oursystem. The ratio A4 /A2 as function of crystal thickness due to FP effects�solid line, right axis�. �b� Left axis: Raman spectrum for a 3 QL thickcrystal �linesymbols� and the Lorentzian corresponding to the emergentmode �dotted line�. Right axis: optical conductivity data �solid line� of abulk Bi2Se3 single crystal taken at T=5 K.

FIG. 3. �Color online� Raman intensities as function of crystal thickness at175 cm−1 �squares� and calculated Raman intensities due to FP interferenceeffects �solid line: optical constants modified from bulk values�.

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observe. Therefore, the emergent mode is intrinsic to Bi2Se3

nanocrystals and not caused by FP effects. However, Fig. 3shows that the change in the overall Raman signal with crys-tal thickness is explained by FP interference effects only ifthe optical constants are modified from the bulk values. Thismodification is not unreasonable given recent photoemissionexperiments.14,16 Interestingly, these data also show the util-ity of Raman measurements, as the overall intensity can beused to determine the thickness of the Bi2Se3 nanocrystals.

While the FP interference described above can accountfor the overall intensity- thickness trends in Fig. 1, the originof the 158 cm−1 mode remains unclear. Interestingly, an ad-ditional mode also appeared in Raman spectra of nanocrys-tals of the isostructural topological insulator Bi2Te3.8 Thismode matched perfectly the frequency of an IR-active modeand so it was attributed to the breaking of inversion symme-try. Shahil et al. suggested that mechanical exfoliation re-sulted in breaks within a QL as well as between them. Asimilar explanation may be appropriate for Bi2Se3: a recentIR reflectance study of the bulk revealed a mode with thesame frequency and width27 �see Fig. 2�b��. However, inBi2Te3 this mode appeared in crystals thinner than 84 nm,whereas it only appears in Bi2Se3 nanocrystals thinner than10 nm. We believe the mode may be due to the built-inelectric fields at the surface. Specifically, the band bendinginherent to materials with surface states will generate anelectric field that will break the inversion symmetry.

We have shown that Raman spectroscopy is an effectivenanometrology tool for identifying nanocrystals of the topo-logical insulator Bi2Se3. This is accomplished by monitoring�i� the overall intensity of the Raman signal and/or �ii� thestrength of the emergent mode at 158 cm−1. The overallthickness dependence of the intensity can be accounted forby proper modeling of the effect of interference on the Ra-man spectra. The origin of the emergent mode remains un-clear, although the presented optical conductivity data sug-gests that inversion symmetry breaking leads to an IR modebecoming Raman active. However, it is interesting to notethat the emergent mode appears for the same thickness re-gime of Bi2Se3 for which a gap has been theorized to opendue to coupling of the two surfaces.14,16 In addition, the polarsurface of Mica may lead to band bending, which breaksinversion symmetry. Therefore performing Raman spectros-copy on a suspended crystal would provide conclusive deter-mination of the influence of the Mica substrate on the Bi2Se3nanocrystals. Nonetheless, we have provided a path for fab-ricating and identifying Bi2Se3 nanocrystals through thecombination of mechanical exfoliation on transparent sub-strates and the use of Raman spectroscopy. This work pavesthe way for future devices and studies of the surface states oftopological insulators.

We are grateful for numerous discussions with Y. B. Kimand H. Y. Kee and we thank Y. J. Choi for the transportmeasurement. Work at the University of Toronto was sup-ported by NSERC, CFI, and ORF; work at Rutgers Univer-sity was supported by the NSF under Grant No. NSF-DMR-0804109.

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Supplemental material corresponding to manuscript: Fabrication and Characterization of TopologicalInsulator Bi2Se3 Nanocrystals

S.Y.F. Zhao1, C. Beekman1, L.J. Sandilands1, J.E.J. Bashucky1, D. Kwok2, N. Lee2, A.D. LaForge3, S.W.Cheong2 and K.S. Burch1a)

1Department of Physics & Institute of Optical Sciences, University of Toronto, 60 St. George Street, Toronto,ON M5S 1A72Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University,136 Frelinghuysen Road, Piscataway, NJ 08854, USA.3Department of Physics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064,USA.

Exfoliation. Bulk Bi2Se3 crystals with structure shown inFig. S1 were prepared as described elsewhere1. The crys-

FIG. S1. Crystal structure of Bi2Se3. A QL consists of a stack of five planesSe(2)-Bi-Se(1)-Bi-Se(2). Within the planes atoms have a triangular lattice(HCP) with three possible positions of the plane (A, B and C). The QLs areseparated by van der Waals bonds (dashed lines) between Se(2)-Se(2) planes.

tals were then mechanically exfoliated onto a Mica substrate(Structure Probe Inc, muscovite V1), in a manner similar tothe preparation of graphene an B22122. Mica was chosenfor exfoliation since it is optically transparent and providesatomically flat, polar surfaces. This leads to an enhancedyield and eases the identification of nanocrystals. However,to maintain the pristine surface of the Mica, exfoliation wasperformed in a sealed box under an Argon atmosphere. Thismaximized yield of thin crystals and minimized the adhesionof contaminants on the substrate surface. To further minimizewater adhesion, substrates were heated to 150◦C before exfo-liation, and Bi2Se3 was deposited onto a freshly cleaved layerof Mica.

Optical and Atomic Force Microscopy. A typical opticalmicroscope image is shown in Fig. S2a, which demonstratesthe high yield of crystals that is achieved with our exfoliationmethod. More importantly, ultrathin crystals with a thicknessof only a few QLs, were found regularly and crystals down toapproximately 2 QL showed optical contrast in transmission.The thickness of the nanocrystals was subsequently measuredvia contact mode atomic force microscopy (AFM), an exam-

a)Electronic mail: kburch@physics.utoronto.ca

ple image is shown in Fig. S2b. In Fig. S2c a profile mea-sured along the line indicated in Fig. S2b is shown, clearlyindicating the structural integrity of the nanocrystals. In addi-tion we have performed scanning electron microscopy (SEM)and energy dispersive X-ray spectroscopy (EDX) to confirmthe integrity of our nanocrystals.

FIG. S2. (a) Optical microscope image of nanocrystals in transmissionmode, the scale bar is 50 µm. Black regions correspond to thick crystals,with thinner nanocrystals appearing grey. The red box indicates the regionimaged by AFM. (b) AFM image of the 3QL crystal with the correspondingcolor bar and the scale bar is 3 µm. (c) Profile across the 3QL crystal, cutalong the line shown in B. (d). Histogram of the height distribution aroundthe edge of the 3QL crystal (shown in B). Crystal thickness is determinedfrom the separation between the observed peaks, which in this case is 2.7 nm.

The nanocrystal thickness was determined through AFM bytaking a histogram of the height distribution around the edgeof the crystal (Fig. S2d). Such histograms show two peaksand crystal thickness is determined from the distance betweenthe observed peaks. For the crystal shown in Fig. S2b this is2.7 nm (3QL).

Raman Spectroscopy. The Raman spectra were takenin a backscattering configuration with a Horiba Jobin YvonLabRam microscope with a 532 nm excitation source and a100x objective (0.8 NA) resulting in an ≈ 1 µm spot size.

1

In accord with previous experiments on exfoliated Bi2Te33,4,we found Bi2Se3 nanocrystals burn easily with laser expo-sure, and a careful study of laser powers and exposure timesto minimize damage and maximize signal to noise ratios wasperformed. Highly reproducible spectra result from a powerof 15 µW with exposure times of 2 minutes and an equivalenttime without exposure, to allow the sample to cool.

Scanning Electron Microscopy (SEM) and Energy Dis-persive X-ray Spectroscopy (EDX) In addition we alsopresent the SEM and EDX characterization of the Bi2Se3nanocrystals. The SEM images (Figs. S3a and B, Vacl = 2kV and Ibeam = 20 µA) show the high yield and integrity ofthe crystals. The integrity is confirmed by the EDX charac-terization (Fig. S3c, black line) which shows that the crystalscontain both Bi and Se with their ratio equal to 0.3. Since weperformed standardless EDX it is not possible to accuratelydetermine stoichiometry from the spectrum. However, com-paring this spectrum to EDX done on a bulk Bi2Se3 singlecrystal (also plotted in Fig. S3c, red line) shows that the ratio

between the Bi and Se peaks is the same for both bulk an thenanocrystal (for Vacl = 5 kV). It also clearly shows the disap-pearance of contributions from the Mica substrate (O, Al, Sipeaks) going from the thin crystal to the bulk.

1 J. G. Analytis, J.-H. Chu, Y. Chen, F. Corredor, R. D.McDonald, Z. X. Shen, I. R. Fisher Phys. Rev. B, 81, 205407(2010)

2 L. J. Sandilands, J. X. Shen, G. M. C. F. Zhao, S. Ono, Y.Ando, K. S. Burch, Phys. Rev. B, 82, 064503 (2010)

3 K. M. F. Shahil, M. Z. Hossain, D. Teweldebrhan, A. A.Balandin, Appl. Phys. Lett.,96, 153103 (2010)

4D. Teweldebrhan, V. Goyal, A. A. Balandin Nano Lett., 10,1209 (2010)

2

m

1 2 3 40.0

0.4

0.8

1.2

1.6 thin crystal bulk

Bi

Al

Si

Se

C

Cou

nts

Energy (keV)

O

(c)

150 200 250 300 350 400

data Fit Peak 1 Fit Peak 2 Fit Peak 3 Fit Peak 4 total fit (4pk) total fit (3pk)

Ram

an In

tens

ity (a

rb)

Raman shift (cm-1)

(d)

(b)(a)

m m

FIG. S3. (a) Scanning electron microscope image of Bi2Se3 nanocrystals, the scale bar is 40 µm. (b) Scanning electron microscope image of 4 QL crystal, thescale bar is 10 µm. (c) Energy dispersive X-ray spectrum of the bulk single crystal (red line) and the nanocrystal (black line) shown in (b). (d). Raman spectrumof the 4 QL region also showing a fit consisting of 3 (gray dashed line) and 4 lorentzians (red solid line).

3