Synthesis of quantum-confined CdS nanotubes

a r X i v :0712.3534v 2 [c o n d -m a t .m t r l -s c i ] 30 J u n 2008

Synthesis of quantum-con?ned CdS nanotubes

A.K.Mahapatra ?

Institute of Physics,Sachivalaya Marg,Bhubaneswar,751005,India

CdS nanotubes with wall thickness comparable to excitonic diameter of the bulk material are syn-thesized by a chemical route.A change in experimental conditions result in formation of nanowires,and well-separated nanoparticles.The diameter and wall thickness of nanotubes measured to be 14.4±6.1and 4.7±2.2nm,respectively.A large number of CdS nanocrystallites having wurzite structure constitute these nanotubes.These nanotubes show high energy shifting of optical absorp-tion and photoluminescence peak positions,compared to its bulk value,due to quantum con?nement e?ect.It is proposed that nucleation and growth of bubbles and particles in the chemical reaction,and their kinetics and interactions are responsible for the formation of nanotubes.

PACS numbers:61.46.+w;81.10.Dn;78.55.Et;78.67.Ch

Introduction

The discovery of carbon nanotubes (Iijima 1991)has generated considerable research interest to synthesize such type of tubular nanostructures of other materials and study its properties.Carbon nanotubes are concep-tualized as the wrapping of graphite layers into a seam-less cylinder.Synthesis of single crystalline nanotubes of other similar type of layered materials like BN (Chopra et al.1995),MoS 2(Feldman et al.1995)and W S 2(Tenne et al.1992)are reported.In these materials,there ex-ists a strong force within the layer plane,but a weak van der waals force between the inter-layer planes.It helps such materials to self assemble in the form of nan-otubes.However,comparatively more isotropic materi-als like CdS (Zhan et al.2000),CdSe (Duan and Lieber 2000),GaAs (Duan et al.2000),and Si (Yu et al.1998)tend to form nanowires instead of nanotubes.There-fore,nanotubes of these isotropic materials are generally synthesized by using nanowires as templates.CdS nan-otubes are also synthesized by using Sn nanowires as the template (Hu et al.2005).However,those synthesized CdS nanotubes have a wall thickness much larger than the excitonic diameter of the bulk CdS (6nm).Hence,no quantum con?nement e?ects could be observed.Aspect ratio and wall thickness of nanotubes play a major role in its mesoscopic properties (Masale et al.1992).The quan-tum con?nement e?ect can be observed if the wall thick-ness of nanotube is comparable to the excitonic diameter of bulk material.It should be noted that quantum con-?nement e?ects are least studied in one-dimensional sys-tems,particularly with tubular structure,as compared to other low-dimensional systems.

The present work reports a single-step chemical pro-cess to synthesize micron length CdS nanotubes with wall thickness comparable to the excitonic diameter of the bulk material.A change in experimental condi-

(c)

( d )

FIG.1:TEM micro-graphs of CdS nanotubes(a)with lower magni?cation(b)with higher magni?cation.(c)Histogram of outer diameter of nanotubes(d)histogram of thickness of nanotubes.

Results and discussions

The formation of CdS nanotubes is con?rmed by trans-mission electron microscopy(TEM).The typical TEM micrographs at

a lower and at a higher magni?cation are shown in Fig.1a,b,respectively.From Fig.1a,it is not possible to conclude whether these are nanotubes or nanowires.However,it is conformed that these nanos-tructures are of micron lengths and have high aspect ra-tio.With higher magni?cation(Fig.1b),a contrast be-tween the solid side wall(darker contrast)and hollow middle part(lighter contrast)of these one-dimensional structure is observed.It gives the signature of forma-FIG.2:TEM micro-graphs of(a)a nanotube with smaller diameter(b)a nanotube with bigger diameter.

tion of nanotubes.Outer diameter and wall thickness of several nanotubes are measured.A histogram of outer diameter of CdS nanotubes along with a?tted Gaussian ,is shown in Fig.1c.The mean diameter of the nanotube is14.4nm with a standard deviation of6.1nm.The his-togram of wall thickness,along with a?tted Gaussian,is shown in Fig.1d.The mean wall thickness of the nan-otubes is4.7nm with a standard deviation of2.2nm. TEM image of a nanotube with a smaller diameter and a nanotube with a larger diameter are shown in Fig.2 a,b respectively.The?uctuation observed in the thick-ness of wall is comparatively less than the?uctuation in diameter of the nanotubes.

The electron di?raction(ED)pattern is shown in Fig. 3a.The rings in ED pattern suggests polycrystalline nature of the nanotubes.These nanotubes are not sin-gle crystalline,but consists of numerous nanocrystallites as it can be seen in high resolution electron micrograph (Fig.3b).It seems large number of nanocrystallites bind together and form nanotube.X-ray di?raction(Fig.4) peaks obtained are also broad due to small dimensions of these constituent crystallites.The XRD pattern shows characteristic peaks of wurzite structure of CdS(JCPDS card No.41-1049).The interplanar distance of0.24nm, as shown in Fig.3b,matches for(102)planes and rings in ED pattern corresponds to(110)and(302)planes of wurzite structure of CdS.The formula used to calculate d-value in the ED pattern is d hkl=λL/R.where L is the Camera length,R is the radius of the ring andλis the wavelength of the electron beam.

The general chemical equation for the formation of CdS nanotube can be written as follows(Pavaskar et al. 1977;Hariskos et al.2001):

CdSO4+4(NH3)→[Cd(NH3)4]SO4

C(NH2)2S+OH?→CH2N2+H2O+HS?

CH2N2+H2O→(NH2)2CO

(NH2)2CO+2OH?→CO2?3+2NH3(g)

HS?+OH?→S??+H2O

[Cd(NH3)4]+++S??→CdS(s)+4NH3(g)

FIG.3:(a)Di?raction pattern of CdS nanotubes (b)HRTEM

The chemical reaction mentioned above,leads to su-persaturation of CdS concentrations.According to classi-cal nucleation theory (Markov 1995),the energy required to form a critical particle homogeneously is given by:

W p =

1

3

σA =

16πσ3k 2

4 and the frictional force due to viscosity of the medium.

This leads to a terminal velocity at which these colloids

will move and is given by the equation(Lamb1945)

V=2

η

·

η+η′

3πηr

)t(5)

The average squared displacement is inversely propor-tional to the radius of the colloid and to the viscosity of the medium.Hence,di?usive motion dominates the motion of the particles and bubbles.These particles and bubbles encounter during their random motion inside the solution,and during this process,the particles get at-tached with the bubbles.It should be noted that particle-bubble attachment can occur when particle-bubble con-tact time is longer than the induction time(Dai et al. 1999).Hence,reduction in induction time enhance the attachment e?ciency.As bubbles and particles are very small in size,the induction time is very small and at-tachment e?ciency is very high.The induction time is de?ned as the time for the liquid?lm between the particle and the bubble to thin,rupture and form a equilibrium three-phase contact.

It is observed that CdTe nanoparticles spontaneously aggregate into a pearl-necklace like structure upon con-trolled removal of the protective shell of organic stabi-lizer,and subsequently recrystallize into nanowires(Tang et al.2002).These CdTe nanoparticles have a large dipole moment and the dipole-dipole interaction be-tween them is responsible for their unidirectional self-organization(Sinyagin et al.2005).CdS nanoparticles with wurzite crystal structure also have a large dipole moment(Sinyagin et al.2005;Blanton et al.1997; Shanbhag and Kotov2006).Hence,particle-attached-bubble as a whole may has a strong net dipole moment; and dipole-dipole interaction between these particle-attached-bubbles is primarily responsible for their uni-directional aggregation.The adjacent nanoparticles on the bubble surface probably get attached at a planar in-terface,reduce total surface energy and transform into a stable nanotube.In the aggregation-based crystal growth (Ban?eld et al.2000;Penn and Ban?eld1998),ran-dom force imparted on the nanoparticles by the medium molecules helps them in rotation and attachment at a planar interface so that they can share a common crys-tallographic orientation.However,even a small misori-entation can lead to dislocation at the interfaces.As the medium is viscous and CdS nanoparticles are attached on a curved surface with equilibrium three phase con-tact,particles could not perfectly orient and attach in a atomically?at interfaces to give a dislocation free single crystalline nanotube.However,annealing after synthesis can help in improving the crystallinity.

It should be noted that colloids after nucleation goes through subsequent growth dynamics along with the above mention steps like bubble-particle attachment and unidirectional aggregation of particle-attached-bubbles. The colloids smaller than its critical size dissolve as sur-face energy is large.Colloids bigger than the critical size only grow.The attachment of colloids also reduces the total surface energy and e?ective size of the colloid become more than its critical radius.Hence,formation of nanotubes with a high aspect ratio are favorable and stable.In the TEM measurement,it is observed that inner wall of nanotubes are comparatively less smooth than the outside wall.A TEM micrograph of a nanotube in a formative stage,in which bubbles are coalescing,is shown in?gure(Fig.5a).Other possible nanostructures like nanoparticles,nanowires,nanowires with spherical cavity(Fig.5b)are also observed in TEM measurements, although they are few in number.These observations lead to believe that bubbles are responsible for hollow-ness inside the nanotube.

In order to con?rm the role of PVA,experiments are carried out in similar experimental condition by not using PVA,and by using much higher amount of PVA(25mL of20%aquous solution).Nanowires are seen in TEM measurements instead of nanotubes,when PVA is not used in the experiment.The typical TEM micrographs at a lower and at a higher magni?cation are shown in Fig.6a,b respectively.In this case probably bubbles did not nucleate.Nanoparticles get aligned unidirectionally due to their dipole moment and nanowires are formed. Formation of nanowires even in the absence of PVA ex-clude the possibility that linear chain structure of poly-mer is someway acting as a template for unidirectional aggregation of the nanoparticles,and forming nanotubes.

5

FIG.5:TEM micrographs of (a)a nanotube in the formative stage (b)a nanowire with a rounded bulge.

When excess amount of PVA is used,the solution be-comes very viscous.Only after ≈4h (in contrast to ≈10min),the solution becomes pale yellow color.The solu-tion then kept for another 90min,and then spin-coated on a Cu-grid to carry out TEM measurements.Even though TEM is carried out by taking the grid on a liq-uid nitrogen cooled sample stage,the PVA matrix forms voids due to the heating e?ect of electron beams.How-ever,it can be seen that nanocrystals are well separated.With too much increase in viscosity,the decrease

in di?u-sive length of nanocrystals is signi?cant (see Eq.5).So nanocrystals could not aggregate to form nanowires or nanotubes.A typical TEM micrograph is shown in Fig.7.It should be noted that well separated HgS nanocrys-tals are also synthesized by a similar chemical procedure using PVA (Mahapatra and Dash 2006).

Most of the interesting properties exhibited by semi-conducting nanomaterials are attributed to quantum con?nement e?ect.The electronic energy levels are strongly dependent on the size and also on the shape of the nanostructure (Kayanuma 1991).In the one-dimensional systems charge carriers are con?ned in two

FIG.6:TEM micro-graphs of (a)CdS nanowire with lower magni?cation,and (b)a single nanowire with higher magni-?cation.

FIG.7:TEM micro-graph of CdS nanocrystals.

dimensions and free in one dimension.The spatial con-?nement of carriers leads to band gap widening and most directly realized by a high energy shift in optical absorp-tion and photoluminescence peak.Quantum con?nement e?ect for zero-dimensional system is studied both the-oretically(Kayanuma1988)and experimentally(Voss-meyer et al.1994)in detail.However,it is not well un-derstood for one-dimensional systems,particularly with tubular structure.

The optical absorption spectrum and photolumines-cence(PL)spectrum of CdS nanotubes are shown in Figs.8,9,respectively.An excitonic peak appears at 464nm in the optical absorption spectrum.The peak position is blue shifted by48nm from its bulk band gap value(512nm).The experimental PL data is?tted as the sum of two Gaussian functions,among which one is due to incident line.The PL peak is best?tted with the Gaussian of492nm mean and26nm standard deviation. It should be noted that,the PL peak position is also blue shifted from its bulk band gap value.High energy peak shifting in the optical absorption and PL spectra are ex-pected due to quantum con?nement e?ect as nanotube wall thickness is comparable to excitonic diameter of bulk CdS.

Luminescence that are observed in semiconductor nanomaterials are generally excitonic and trapped emis-sions.Excitonic emission is sharp and can be ob-served near the absorption edge if the material is pure (Pankove1975).However,if the material is impure or o?-stoichiometric then a broad and intense emission oc-cur at higher wavelength due to recombination of charge carriers at trapped states.Hence the band edge lumines-cence that appears at492nm is due to excitonic tran-sition.Band edge luminescence at470nm is also ob-served for CdS nanotubes synthesized by sacri?cial tem-plate method(Li et al.2006).However,CdS nanotubes perimental data is?tted with Gaussian functions. synthesized by sacri?cial template method also show a very intense and broad peak centered around560nm due to presence of trapped states.No such peak is ob-served in our synthesized CdS nanotubes.This suggests purity and stoichiometric nature of our synthesized CdS nanotubes.

Excitonic transition in a semiconductor can be ob-served well only at low temperature.However,it can be observed even at room temperature due to enhance-ment of oscillator strength in the low-dimensional sys-tems(Kayanuma1991).It should be noted that excitonic transitions is not observed in optical absorption and PL measurements for bulk CdS which was prepared by the similar chemical route(Pavaskar et al.1977).However,a clear excitonic feature arises in both absorption and PL measurements when it forms nanotubes.The PL peak is red shifted by150meV with respect to absorption peak.Such a large red shift,known as stokes shift,of 147meV is also reported for CdS nanoparticles(Tam-borra et al.2004).The di?erence in the absorption and emission states avoid sample self-absorption and could be very useful in making LEDs(Sze1981).

Conclusion

CdS nanotubes with wall thickness comparable to ex-citonic diameter of the bulk material are synthesized by a chemical synthesis process.These synthesized nanotubes show band gap widening and enhanced oscillator strength due to quantum con?nement e?ects.A large stokes shift of150meV is also observed.Bubbles are responsible for the hollowness of nanotubes;and bubbles of dissolved gases can be utilized to make nanostructures with hollow interior.

7

Acknowledgment

The help and encouragement received from Dr S.N. Sahu is gratefully acknowledged.Mr A.K.Dash,Mr U. M.Bhatta,and Dr P.V.Satyam are acknowledged for their help in TEM measurements.

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