Journal of Luminescence 132 (2012) 2114–2121

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

http://d

n Corr

Xuefu A

fax: þ8

E-m

journal homepage: www.elsevier.com/locate/jlumin

Observations of energy transfer and anisotropic behavior in ZnOnanoparticles surface-modified by liquid-crystalline ligands

Fan Li a,b,n, Qiujuan Li a,b, Yiwang Chen a,b,c,n

a Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, Chinab Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, Chinac Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

a r t i c l e i n f o

Article history:

Received 13 January 2012

Received in revised form

5 March 2012

Accepted 26 March 2012Available online 3 April 2012

Keywords:

ZnO nanoparticles

Photoluminescence

Liquid crystals

Anisotropic

13/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2012.03.071

esponding authors at: Institute of Polymer

venue, Nanchang 330031, China. Tel.: þ86 7

6 791 83969561.

ail addresses: lfan@ncu.edu.cn (F. Li), ywchen

a b s t r a c t

Here we report the fabrication of a novel nano-level hybrid of ZnO nanoparticles (NPs) and liquid

crystals (LCs) by the attachment of organic LC molecules with a mercapto moiety, namely, 40-n-(6-

mercaptohexyloxy)-4-cyanobiphenyl (6CNBP-SH), to the surface of ZnO nanoparticles. The dispersion

of modified ZnO NPs (6CNBP-SH@ZnO) is greatly improved by the surface modification of 6CNBP-SHligands. The photoluminescence (PL) measurement shows that the ultra-violet emission of ZnO can be

enhanced by the surface modification of 6CNBP-SH ligands and annealing at liquid crystal state

temperature of 6CNBP-SH@ZnO (110 1C). Meanwhile, defect-related emission of ZnO in 6CNBP-SH@ZnO almost disappears. We attribute this observation to the energy transfer between the ZnO

NPs and 6CNBP-SH, surface passivation of the ZnO and formation of ZnO nano-dispersing structure

induced by 6CNBP-SH molecules. The anisotropic behavior of 6CNBP-SH@ZnO is also investigated. The

results indicated that the 6CNBP-SH liquid-crystalline ligands could endow the 6CNBP-SH@ZnO hybrid

obvious mesoscopic behavior. In addition, the increased optical anisotropy of 6CNBP-SH@ZnO is also

observed upon thermal treatment at 110 1C.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The inorganic semiconducting nanoparticles have been thefocal point of research in the past decade. Their unique propertieslike quantum confinement, tailoring of band gap and solutionprocessability make them ideal candidates for application infrontier areas of research such as photovoltaics [1–4], catalysis[5,6], light-emitting diodes [7], biological fluorescent labels [8,9],lasers [10] and drug delivery [11]. Among all the inorganicsemiconducting nanoparticles, zinc oxide nanoparticles (ZnONPs) have attracted increasing attention because ZnO NPs canbe easily synthesized and ZnO is a ‘‘green’’ material that isbiocompatible, biodegradable, and nontoxic for medical applica-tions and environmental science [12]. In addition, ZnO is a wideband gap (3.37 eV) semiconductor having a high electron–holebinding energy (60 meV). Owing to their excellent opticaland electrical properties, ZnO NPs have become predominantsemiconductor materials for nanoscale devices, such asnano-generators [13], gas sensors [14], highly efficient solar cells,

ll rights reserved.

s, Nanchang University, 999

91 83969562;

@ncu.edu.cn (Y. Chen).

field-emission transistors [15], ultraviolet photodetectors [16],and biomedical systems [17].

Liquid crystals (LCs) are finding increasing applications in awide variety of fields, including liquid-crystal display technology,materials science, bioscience, etc. Liquid crystals offer the possi-bility of creating self-organizing and self-assembling materialswhich possess both order and mobility at molecular, supramole-cular and macroscopic levels [18].

The hybridization of inorganic nanoparticles with liquid-crystal-line materials is an important ongoing research area in modernnanotechnology. Such hybrids exhibit properties originating fromboth components in the system, which in turn give rise to severalapplications in nanoelectronics and photonics [19,20]. Previouslyreported LC hybrids commonly involve gold [21,22], cobalt [23],cadmium selenide [24] and iron oxide NPs [25]. These nanocrystals,surface-modified by liquid crystal molecules, represent an excitingresearch area. These researches demonstrate that the surface mod-ification of nanocrystals using liquid-crystalline molecules couldallow one to couple advantages associated with the inorganicnanocrystals and liquid crystal molecules. What is more, liquidcrystals exhibit molecular orientation in their mesophases, whichprovides a path for electrons or holes to move between electrodes.High-mobility photoconductivity in liquid crystals was discovered in1995 [26,27]. Nanoparticles incorporated in a liquid crystal tend toalign in rows within the liquid crystal [28]. Molecular order in a

BrBr HO CN

BrO CN

K2CO3/Acetone

F. Li et al. / Journal of Luminescence 132 (2012) 2114–2121 2115

liquid crystal/nanoparticle mixture facilitates electron/hole transferin a photovoltaic cell [29].

Inspired by these researches, we rationally apply a liquid-crystalline ligand, 40-n-(6–mercaptohexyloxy)-4-cyanobiphenyl(6CNBP-SH), for the surface modification of ZnO nanoparticlesto fabricate a nano-level hybrid of ZnO and LCs. Here we showthat the 6CNBP-SH ligands bearing mercapto functionalities playa crucial role in anisotropic behaviors of ZnO NPs and conveyunusual optical properties.

AcSK/DMF

H3C SO CN

O

NaOMe/MeOHNH4Cl

HSO CN

ZnO ZnO

= HS O CN

6CNBP-SH

Scheme 1. Synthesis of 40-n-(6- mercaptohexyloxy)-4-cyanobiphenyl ligands

(6CNBP-SH) and schematic diagram of modification mechanism occurring in

2. Experiment

2.1. Synthesis of ZnO nanoparticles

ZnO nanoparticles were prepared using an adapted procedurebased on the work of Pacholski et al. [30]. The general procedureused for the preparation of nanoparticles was as follows. Zincacetate dihydrate (Zn(CH3COO)2 �2H2O, 1.23 g, 5.6 mmol) wasdissolved in methanol (55 mL) at 60 1C; a solution of potassiumhydroxide (KOH, 87%, 0.48 g, 8.57 mmol) in methanol (25 mL)was added to the zinc acetate dihydrate solution under magneticstirring for 20 min. After 5 min, the solution became translucent.After 1.5 h, the nanoparticles started to precipitate and thesolution became turbid. After 2 h and 15 min, the heater andstirrer were removed and the nanoparticles were allowed toprecipitate for an additional 2 h. Precipitate and mother liquorwere separated, and the precipitate was washed twice withmethanol (20 mL). After the washing steps (5 min), the suspen-sion was left unstirred for a minimum of 1 h to reach fullprecipitation. The washed precipitate was treated with chloro-form or chlorobenzene (5 mL) to dissolve the nanoparticles. Thissolution was only slightly translucent, almost transparent, andwas stable for more than 2 weeks.

solution.

2.2. Synthesis of 40-(6-mercaptohexyloxy)-4-cyanobiphenyl

(6CNBP-SH)

The liquid crystalline ligand, 40-n-(6–mercaptohexyloxy)-4-cyanobiphenyl (6CNBP-SH), was synthesized from 40-hydroxybi-phenyl-4-carbonitrile via 3 steps (see Scheme 1), according to areference method [21]. 40-n-(6–thioacetic acid hexyloxy)-4-cya-nobiphenyl (0.4833 g), sodium methoxide (0.2217 g), ammoniumchloride (0.2195 g) and methanol (125 mL) were added togetherin a 250 mL round bottom flask. The reaction system was stirredfor 24 h at 75 1C. The reaction mixture was then filtered toremove the saline solid matter under reduced pressure to get anorganic phase. Solvent in the organic phase was removed byreduced pressure and the residue was purified by column chro-matography (SiO2, 2:1 dichloromethane-hexanes) to give0.3114 g of the desired material (white powder).

2.3. Hybridization of ZnO nanoparticles and 6CNBP-SH

Hybridization of ZnO nanoparticles and 6CNBP-SH was carriedout as follows. 6CNBP-SH was dissolved in chloroform (CHCl3)solution. The 6CNBP-SH solution was dropwise added to the ZnOnanoparticles (8.75 mg/mL) solution under magnetic stirring(6CNBP-SH/ZnO¼1/1, mol/mol). The sample was labeled as6CNBP-SH@ZnO. The solution was then left to sonicate for30 min at room temperature. 6CNBP-SH@ZnO was purified bycentrifugation twice in CHCl3 solution. This step was repeatedtwice in order to remove the excess ligand molecules, and thenthe 6CNBP-SH@ZnO was re-dispersed in CHCl3.

2.4. Characterizations

The nuclear magnetic resonance (NMR) spectra are collected on aBruker ARX 400 NMR spectrometer with deuterated chloroform asthe solvent and with tetramethylsilane (d¼0) as the internalstandard. The infrared (IR) spectra are recorded on a ShimadzuIRPrestige-21 Fourier transform infrared (FT–IR) spectro-photometerby using KBr substrates. The ultraviolet–visible (UV–vis) spectra ofthe samples are recorded on a Perkin Elmer Lambda 750 s spectro-photometer. Fluorescence measurement for photoluminescence (PL)of all simples is carried out on a Shimadzu RF-5301 PC spectro-fluorophotometer with a xenon lamp as the light source. Differentialscanning calorimetry (DSC) is used to determine phase-transitiontemperatures on a Perkin-Elmer DSC 7 differential scanning calori-meter with a constant heating/cooling rate of 10 1C/min. Textureobservations by polarizing optical microscopy (POM) are made witha Nikon E600POL polarizing optical microscope equipped with anInstec HS 400 heating and cooling stage. X-ray diffraction (XRD)study of the samples was carried out on a Bruker D 8 Focus X-raydiffractometer operating at 30 kV and 20 mA with a copper target(l¼1.54 A) and at a scanning rate of 21/min. Atomic force micro-scopic (AFM) images are measured on a nanoscope III A (DigitalInstruments) scanning probe microscope using the tapping mode.

3. Results and discussions

The chemical structure of the 6CNBP-SH is characterized byFourier transform infrared (FT-IR) and 1H NMR analysis. The FT-IR

F. Li et al. / Journal of Luminescence 132 (2012) 2114–21212116

spectrum of the 6CNBP-SH is shown in Fig. 1A. 6CNBP-SHincludes the following main characteristic absorption bands:3030 cm�1 band of the C–H stretching mode of benzene ring

2

4

6

8

10

Wei

ght R

emai

ning

(%)

4000

O-HZn-O

CNSHCH2

ZnO

6CNBP-SH

ZnO@6CNBP-SH

8.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0

ab

c

de

f

gh h'

j j' kk' i'i

2

4

6

8

10

O-HZn-O

CNSHCH

ZnO

6CNBP-SH

ZnO@6CNBP-SH

Wavenumber (cm-1)

8.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0PPm

ab

c

de

f

gh h'

j j' kk' i'i

3500 3000 2500 2000 1500 1000 500

Fig. 1. (A) FI-IR spectra of the pristine ZnO NPs, 6CNBP-SH and 6CNBP-SH@ZnO. (B) TG

spectrum of 6CNBP-SH in CDCl3. (D) 1H NMR spectrum of 6CNBP-SH@ZnO in CDCl3.

6CNBP-SH@ZnO

6CNBP-SH@ZnO

Fig. 2. (A) Optical image of chloroform solutions containing pristine ZnO nanoparticles

8.75 mg/mL. (B) AFM micrograph of 6CNBP-SH@ZnO. (C) AFM micrograph of pristine

and 1600, 1500 and 830 cm�1 bands of symmetric benzeneskeletal vibration. In addition, 2850 cm�1 band of the S–Hstretching mode of mercapto, 2225 cm�1 band of CRN,

0

0

0

0

0

0ZnO, 93.54%

6CNBP-SH, 2.92%

6CNBP-SH@ZnO, 67.38%

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

1000

0

0

0

0

0

Temperature (°C)

ZnO, 93.54%

6CNBP-SH, 2.92%

6CNBP-SH@ZnO, 67.38%

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5PPm

200 300 400 500 600 700

A thermograms of pristine ZnO NPs, 6CNBP-SH and 6CNBP-SH@ZnO. (C) 1H NMR

ZnO NPs

ZnO

(right) and 6CNBP-SH@ZnO (left). The concentration of ZnO NPs in both cases is

ZnO NPs.

0.0

0.5

1.0

1.5

2.0

2.5

Abs

orba

nce

ZnO NPs 6CNBP-SH@ZnO 6CNBP-SH

0

100

200

300

400

500

600

PLE

Inte

nsity

40ZnO

F. Li et al. / Journal of Luminescence 132 (2012) 2114–2121 2117

1250 cm�1 band of C–O–C, and 2930 cm�1 band of CH2 areobserved. Meanwhile, the 1H NMR analysis of 6CNBP-SH is shownin Fig. 1C; the aromatic H resonance peaks appear in the lowmagnetic region (d¼7.05–7.84), whereas the aliphatic H reso-nance peaks appear in the high magnetic region (d¼1.42–4.06).The peak of the near-neighboring aliphatic H to the –O– shifts tothe middle region (d¼4.06). The resonance peak of the mercapto(–SH) appears at d¼1.5. All results can indicate that 6CNBP-SH issuccessfully synthesized.

The FT-IR spectrum of ZnO (Fig. 1A) shows main absorptionbands at 3438, 1577, and 1384 cm�1, which correspond to theO–H mode,as well as asymmetric and symmetric CQO stretchingmodes of zinc acetate, respectively. The peak at 431 cm�1 is thecharacteristic absorption of Zn–O bond. The 6CNBP-SH@ZnOhybrid is successfully prepared, which can be demonstrated byFT-IR spectroscopy. The FT-IR spectrum of 6CNBP-SH@ZnOincludes the characteristic absorption peaks of ZnO NPs and6CNBP-SH. The result has been further strengthened by 1H NMRexperiment of 6CNBP-SH@ZnO (Fig. 1D). The resonance peaks of6CNBP-SH@ZnO are almost the similar to the resonance peaks of6CNBP-SH.

Thermogravimetric analysis (TGA) can not only confirm thatthe 6CNBP-SH molecules have been successfully attached ontothe surface of ZnO nanoparticles, but also quantify the 6CNBP-SHcover on the surface of ZnO nanoparticles. The 6CNBP-SH@ZnOsamples show significant mass loss in the 250–400 1C range, dueto the degradation of 6CNBP-SH. Thus, we can calculate that theaverage weight percentage of 6CNBP-SH in the 6CNBP-SH@ZnOis 28.9 wt% [31]. Such results indicate that the 6CNBP-SH mole-cules have been successfully attached onto the surface of ZnOnanoparticle.

Fig. 2 shows an optical photograph of the chloroform solutionsof pristine ZnO NPs and 6CNBP-SH@ZnO, which also can be usedto demonstrate the successful surface modification of ZnO NPs by6CNBP-SH ligands. Prior to adding 6CNBP-SH into ZnO nanopar-ticles, ZnO nanoparticles form aggregates in solution as indicatedby strong light scattering of the solution. When 6CNBP-SH ligandsare attached to the surface of ZnO NPs, the solubility of the6CNBP-SH@ZnO increases dramatically. We attribute this phe-nomenon to improved dispersion and uniformity of ZnO NPs dueto 6CNBP-SH liquid crystal molecules. This result is in agreementwith three-dimensional images of the atomic force microscope(AFM) in Fig. 2B, C. It is clear that the ZnO NP film reveals anobvious coarse surface with the root mean square (RMS) rough-ness of 55.8 nm at 33.5 mm�33.55 mm scan sizes as shown in

10

b

a

6CNBP-SH@ZnO

ZnO

2θ (Degree)

100

002 10

1

102

110

20 30 40 50 60

Fig. 3. XRD patterns of pristine ZnO and 6CNBP-SH@ZnO.

Fig. 2C, and obvious ZnO NPs aggregation with the size distribu-tion above 170 nm. After the surface modification, the RMSroughness of modified ZnO NPs film obviously decreases to18.2 nm, which means that the uniformity and roughness of theZnO NPs film have a dramatic improvement. The roughness isconsidered to be a signal of the dispersion of the ZnO NPs film[32]. It can be interpreted that the liquid crystalline ligands play acrucial role in the arrangement of nanoparticles [21].

X-ray diffraction (XRD) analysis is performed to investigate thecrystalline structure of pristine ZnO nanoparticles and 6CNBP-SH@ZnO (Fig. 3). In the XRD pattern of the pristine ZnO nano-particle, a series of characteristic peaks at 2y of 31.83 (1 0 0),

2500

5

10

15

20

25

30

35

350

Nor

mal

ized

PL

Inte

nsity

Wavelength (nm)

6CNBP-SH@ZnO

ZnO × 5

PL

Inte

nsity

Wavelength (nm)300 350 400 450 500 550 600

400 450 500 550 600

Fig. 4. Steady-state spectra of pristine ZnO NPs (black), 6CNBP-SH (red) and

6CNBP-SH@ZnO (green). (A) Absorption. (B) PLE at the emission wavelength of

390 nm. (C) PL at the excitation wavelength of 300 nm. (inset of C: Normalized PL

spectra of pristine ZnO and 6CNBP-SH@ZnO). The concentration of ZnO NPs is the

same for all samples. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

F. Li et al. / Journal of Luminescence 132 (2012) 2114–21212118

34.43 (0 0 2), 36.52 (1 0 1), 47.42 (1 0 2), 56.67 (1 1 0) areobserved [33], which are in accordance with the zincite phaseof ZnO (International Center for Diffraction Data, JCPDS 5-0664).

40

102θ (Degree)

6CNBP-SH

HS O CN

T (°C)

Hea

t flo

w e

ndo

second heating

45 50 55 60 65 70 75

20 30 40 50 60

Fig. 5. Mesogenic texture observed by POM for (A) 6CNBP-SH, (B) 6CNBP-SH@ZnO a

nitrogen during the second heating at heating rate: 10 1Cmin�1. (inset of C: XRD patte

3500

200

400

600

800

1000

1200

1400

PL

Inte

nsity

Wavelengt

Un-annealed 66CNBP-SH@Z

400 4

Fig. 6. (A) Photoluminescence spectra for unannealed 6CNBP-SH@ZnO and annealed 6Cand (C) the 6CNBP-SH@ZnO annealed at mesophase state temperature.

Meanwhile, it is observed that the XRD pattern of 6CNBP-SH@ZnO is almost the same as that of the pristine ZnO nano-particles, indicating that 6CNBP-SH liquid crystalline molecules

40

S O CN

T (°C)

Hea

t flo

w e

ndo

60 80 100 120 140 160

nd DSC thermograms of (C) 6CNBP-SH and (D) 6CNBP-SH@ZnO recorded under

rns of 6CNBP-SH).

h (nm)

CNBP-SH@ZnOnO annealed at LC state

50 500 550

NBP-SH@ZnO at LC temperature. AFM images of (B) unannealed 6CNBP-SH@ZnO

0

6

12

18

A⊥

A||

Inte

nsity

Unannealed 6CNBP-SH@ZnO

150

F. Li et al. / Journal of Luminescence 132 (2012) 2114–2121 2119

attached to ZnO NPs do not affect the crystalline structure of ZnOnanoparticles.

In order to get an insight into the photophysics properties ofthe pristine ZnO NPs, 6CNBP-SH and 6CNBP-SH@ZnO, UV–visabsorption spectra and photoluminescence (PL) spectra weremeasured. Fig. 4A shows the UV–vis absorption spectra of pristineZnO nanoparticles, 6CNBP-SH and 6CNBP-SH@ZnO. The max-imum absorption peak of 6CNBP-SH is at �300 nm and theabsorption edge of the ZnO NPs is observed at �389 nm. Forthe absorption spectrum of 6CNBP-SH@ZnO, there are two peakswhich include the characteristic absorption peaks of 6CNBP-SHand ZnO. The PL spectra of the pristine ZnO NPs and 6CNBP-SH@ZnO are shown in Fig. 4C. ZnO NPs show two emission bands,one is the UV band with a peak position at about 392 nm causedby the transition of excitons, and the other is the deep levelemissions located at 540 nm, commonly referred to as the deep-level or the trap-stated emission caused by the defects of thecrystal such as oxygen vacancy (VO), zinc vacancy (VZn), inter-stitial zinc (Zni), etc. [34]. However, it is interesting that, com-pared with the pristine ZnO NPs, UV emission intensity ofmodified ZnO nanoparticles obviously increases. Meanwhile, deeplevel emission of modified ZnO NPs completely disappears (seethe inset of Fig. 4C). It can be attributed to the energy transferbetween ZnO nanoparticles and 6CNBP-SH molecules, and thesurface passivation of 6CNBP-SH molecules, which may at leastpartially eliminate the surface trap states.

This phenomenon of energy transfer is further investigatedand discussed in detail by the PL excitation (PLE) measurement at

90

Abs

orpt

ion

inte

nsity

Polarization absorption angle θ (deg)

polarized absorption

3000.0

0.5

1.0

1.5

Abs

orba

nce

Wavelength (nm)

before annealing

60

Abs

orpt

ion

inte

nsty

Polarized angle θ (deg)

3000.0

0.5

1.0

1.5

A⊥

A⊥

A||

A||

A⊥

A⊥

A||

A||

Abs

orba

nce

Wavelength (nm)

after annealing

400 500 600 700 800

400 500 600 700 800

90 120 150 180 210 240

120 150 180 210 240

Fig. 7. Polarized absorbance spectra of the 6CNBP-SH@ZnO film before and after

annealing at 110 1C on a quartz plate.

the peak emission wavelength of the ZnO NPs of 390 nm (Fig. 4B).Firstly, for wavelengths above 370 nm, where the 6CNBP-SHabsorption is negligible, the absorption spectrum of the 6CNBP-SH@ZnO almost coincides with the spectrum of pristine ZnO NPs,indicating that the intrinsic electronic structure of the ZnO NPs isnot affected by attachment of the 6CNBP-SH. Secondly, forexcitation wavelengths longer than ca. 370 nm only the ZnONPs in the 6CNBP-SH@ZnO are excited and behave identically

0

50

100

A⊥

A||

6CNBP-SH@ZnO annealed at 110°C

Inte

nsity

350

0.1

0.2

0.3

0.4

0.5

Ani

sotro

py

Wavelength (nm)

Unannealed 6CNBP-SH@ZnO 6CNBP-SH@ZnO annealed at 110 °C

400 450 500 550

Fig. 8. (A, B) Photoluminescence polarization spectroscopy for the unannealed

6CNBP-SH@ZnO film and the 6CNBP-SH@ZnO film annealed at 110 1C on a quartz

plate. (C) Polarization anisotropy spectra for the emission from the unannealed

6CNBP-SH@ZnO film and the 6CNBP-SH@ZnO film annealed at 110 1C on a

quartz plate.

F. Li et al. / Journal of Luminescence 132 (2012) 2114–21212120

to pristine ZnO NPs. As the excitation wavelength is reducedbeyond 370 nm, the 6CNBP-SH molecules begin to absorb lightand transfer energy to the ZnO NPs, which results in a higher PLEsignal [35]. The energy transfer process is also evident in the PLspectra at an excitation wavelength of 300 nm. Upon attachmentof 6CNBP-SH to the NPs, the PL of 6CNBP-SH is strongly quenchedand the PL of the ZnO NPs shows a distinct enhancement.

Thermotropic liquid crystals are ‘‘soft materials’’ that can formmobile and ordered states in a certain temperature range. Themesomorphic behaviors of the synthesized 6CNBP-SH are demon-strated using POM, XRD and DSC. The polarized optical micro-scopy texture of the 6CNBP-SH is displayed in Fig. 5A and it canbe observed that the 6CNBP-SH exhibits obvious optical aniso-tropy. From the DSC result, the 6CNBP-SH exhibits a meltingtransition at Tm¼55 1C and a clearing transition at Tc¼67 1C(Fig. 5C). In the XRD pattern of 6CNBP-SH quenched at 55 1C,the 6CNBP-SH can be assigned to a nematic liquid crystal (see theinset of Fig. 5C). Meanwhile, DSC measurement and the POMimage indicate that the 6CNBP-SH liquid-crystalline ligands canendow the 6CNBP-SH@ZnO obvious mesoscopic behavior fromFig. 5B, D.

Comparative study on photoluminescence emission of unan-nealed 6CNBP-SH@ZnO and annealed 6CNBP-SH@ZnO at itsliquid crystal state temperature (110 1C) is reported, as shownin Fig. 6A. The band-edge emission intensity of the annealedsample increases by about 60% compared with that of un-annealed one. This enhancement may be attributed to theformation of ZnO nano-dispersing structure induced by 6CNBP-SH molecules at its liquid crystal state temperature [36]. Themorphologies of 6CNBP-SH@ZnO film before and after annealingat its liquid crystal state temperature (110 1C) are observed byatomic force microscopy (AFM), as shown in Fig. 6B, C. The filmsare prepared on quartz glass substrates by spin coating 6CNBP-SH@ZnO from a low concentrated chloroform solution (8.5 mg/mL). AFM images reveal that the 6CNBP-SH@ZnO film becomessmoother after annealing, attributed to the formation of nano-dispersing structure of ZnO NPs induced by 6CNBP-SH in itsliquid crystal state, which can lead to the improvement of the filmuniformity.

The anisotropic behavior of 6CNBP-SH@ZnO film under themesophasic annealing can exhibit anisotropic optical propertieswhich are manifested by measuring polarized absorption spectraand polarized PL spectra (Figs. 7 and 8). Fig. 7 shows theabsorption spectra of the 6CNBP-SH@ZnO films before and afterannealing at 110 1C with the incident linearly polarized lightparallel (2101 or 1901) or perpendicular (1401 or 1201) to the longaxis direction. More obvious absorption anisotropy is observedafter mesophase annealing (110 1C). In addition, increased PLanisotropy of 6CNBP-SH@ZnO is also observed in the polarizationphotoluminescence spectra of the annealed 6CNBP-SH@ZnOcompared with the unannealed 6CNBP-SH@ZnO, which areshown in Fig. 8. The increase of optical anisotropy in the annealed6CNBP-SH@ZnO reveals the orderly arrangement of ZnO nano-particles induced by liquid crystal molecules 6CNBP-SH.

4. Conclusion

In conclusion, a novel nano-level hybrid of ZnO and liquidcrystals (LCs) was successfully fabricated by the attachment ofliquid-crystalline ligands, 40-n-(6- mercaptohexyloxy)-4-cyanobi-phenyl (6CNBP-SH), to the surface of ZnO nanoparticles. Liquid-crystalline 6CNBP-SH ligands, acting as a functional group, cannot only improve the dispersion and orderly arrangement of ZnOnanoparticles, but also yield a liquid-crystalline hybrid systemwhich displays unusual anisotropic optical properties. Meanwhile,

ultraviolet emission of ZnO NPs has been greatly enhanced andsimultaneously the trap-related broad luminescence intensity canobviously disappear through surface modification using 6CNBP-SHas liquid crystal ligands, attributed to the energy transfer betweenZnO nanoparticles and 6CNBP-SH molecules, ZnO nano-dispersingstructure induced by 6CNBP-SH molecules at LC state temperatureand the surface passivation of 6CNBP-SH molecules. In combinationwith promising optical properties, the nano-level hybridization ofinorganic semiconducting nanoparticles and LCs will make themone of the most advanced functional materials for electro-opticalapplications.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (51073076, 51172103 and 50902067).

References

[1] StevenReferences [1-11] were repeated in the reference list.The first set ofreferences has been retained as it was matching with the citations in thetext.The second set of references has been deleted accordingly.Please checkand correct if necessary.A. McDonald, Gerasimos Konstantatos, Shiguo Zhang,Paul W. Cyr, Ethan J.D. Klem, Larissa Levina, Edward H. Sargent, Nat. Mater. 4(2) (2005) 138.

[2] Wendy U. Huynh, Janke J. Dittmer, A. Paul Alivisatos, Science 295 (5564)(2002) 2425.

[3] V. Prashant, Kamat, J. Phys. Chem. B 106 (32) (2002) 7729.[4] A.J. Nozik, M.C. Beard, J.M. Luther, M. Law, R.J. Ellingson, J.C. Johnson, Chem.

Rev. 110 (11) (2010) 6873.[5] Chun-Wei Chen, Takeshi Serizawa, Mitsuru Akashi, Chem. Mater. 11 (5)

(1999) 1381.[6] Hanrong Gao, Robert J. Angelici, Organometallics 18 (6) (1999) 989.[7] Seth Coe, Wing-Keung Woo, Moungi Bawendi, Vladimir Bulovic, Nature 420

(6917) (2002) 800;Park Jong Hyeok, Kim Jung Yong, Chin Byung Doo, Kim Young Chul, KimJai Kyeong, O.Ok Park, Nanotechnology 15 (9) (2004) 1217.

[8] Marcel Bruchez, Mario Moronne, Peter Gin, Shimon Weiss, A. Paul Alivisatos,Science 281 (5385) (1998) 2013.

[9] Warren C.W. Chan, Shuming Nie, Science 281 (5385) (1998) 2016.[10] V.I. Klimov, A.A. Mikhailovsky, Su Xu, A. Malko, J.A. Hollingsworth,

C.A. Leatherdale, H.-J. Eisler, M.G. Bawendi, Science 290 (5490) (2000) 314.[11] L. Ren, G.M. Chow, Mater. Sci. Eng. C 23 (1-2) (2003) 113.[12] J. Zhou, N.S. Xu, Z.L. Wang, Adv. Mater. 18 (18) (2006) 2432.[13] Xudong Wang, Jinhui Song, Zhong Lin Wang, J. Mater. Chem. 17 (8) (2007)

711.[14] N. Hongsith, C. Viriyaworasakul, P. Mangkorntong, N. Mangkorntong,

S. Choopun, Ceram. Int. 34 (4) (2008) 823.[15] Michael S. Arnold, Phaedon Avouris, Zheng Wei Pan, Zhong L. Wang, J. Phys.

Chem. B 107 (3) (2002) 659.[16] Jin Hyung Jun, Hojun Seong, Kyoungah Cho, Byung-Moo Moon, Sangsig Kim,

Ceram. Int. 35 (7) (2009) 2797.[17] Kumar Nitin, Dorfman Adam, Hahm Jong-in, Nanotechnology 17 (12) (2006)

2875.[18] Takashi Kato, Norihiro Mizoshita, Kenji Kishimoto, Angew. Chem. Int. Ed. 45

(1) (2006) 38;Carsten Tschierske, Chem. Soc. Rev. 36 (12) (2007) 1930.

[19] Sarmenio Saliba, Yannick Coppel, Patrick Davidson, Christophe Mingotaud,Bruno Chaudret, Myrtil L. Kahn, Jean-Daniel Marty, J. Mater. Chem. 21 (19)(2011) 6821.

[20] S.H. Cha, Appl. Phys. Lett. 92 (2) (2008) 023506.[21] Insik In, Young-Wook Jun, Yun Jun Kim, Sang Youl Kim, Chem. Commun. 6

(2005) 800.[22] Hao Qi, Brandy Kinkead, Vanessa M. Marx, Huai R. Zhang, Torsten Hegmann,

Chem. Phys. Chem. 10 (8) (2009) 1211.[23] L. Zadoina, B. Lonetti, K. Soulantica, A.F. Mingotaud, M. Respaud, B. Chaudret,

M. Mauzac, J. Mater. Chem. 19 (43) (2009) 8075.[24] Brandy Kinkead, Torsten Hegmann, J. Mater. Chem. 20 (3) (2010) 448.[25] Kiyoshi Kanie, Atsushi Muramatsu, J. Am. Chem. Soc. 127 (33) (2005) 11578.[26] Matthew T. Lloyd, John E. Anthony, George G. Malliaras, Mater. Today 10 (11)

(2007) 34.[27] Jenny Nelson, Science 293 (5532) (2001) 1059.[28] L.J. Martinez-Miranda, Lynn K. Kurihara, J. Appl. Phys. 105 (8) (2009) 084305;

J.C. Loudet, P. Poulin, Phys. Rev. Lett. 87 (16) (2001) 165503.[29] L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons, R.H. Friend,

J.D. MacKenzie, Science 293 (5532) (2001) 1119.[30] Claudia Pacholski, Andreas Kornowski, Horst Weller, Angew. Chem. Int. Ed.

41 (7) (2002) 1188.

F. Li et al. / Journal of Luminescence 132 (2012) 2114–2121 2121

[31] Qingling Zhang, Thomas P. Russell, Todd Emrick, Chem. Mater. 19 (15) (2007)3712.

[32] Baoquan Sun, Henning Sirringhaus, Nano Lett. 5 (12) (2005) 2408.[33] Waldo J.E. Beek, Martijn M. Wienk, Martijn Kemerink, Xiaoniu Yang,

A.J.Janssen Rene, J. Phys. Chem. B 109 (19) (2005) 9505.

[34] Xi Zhang, Haibo Zeng, Weiping Cai, Mater. Lett. 63 (2) (2009) 191.[35] Andrey A. Lutich, Guoxin Jiang, Andrei S. Susha, Andrey L. Rogach, Fernando

D. Stefani, Jochen Feldmann, Nano Lett. 9 (7) (2009) 2636.[36] Ting-Yu Liu, Hung-Chou Liao, Chin-Ching Lin, Shang-Hsiu Hu, San-

Yuan Chen, Langmuir 22 (13) (2006) 5804.