- Email: [email protected]
Structure and optical properties of pure and doped ZnO 1D nanostructures
Materials Letters 91 (2013) 369–371
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structure and optical properties of pure and doped ZnO 1D nanostructures Weichang Zhou a,n, Dongsheng Tang a, Ruibin Liu b, Bingsuo Zou b,n a
Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, College of Physics and Information Science, Hunan Normal University, Changsha 410081, China School of MSE, Beijing Institute of Technology, Beijing 100081, China
b
a r t i c l e i n f o
abstract
Article history: Received 12 July 2012 Accepted 9 October 2012 Available online 22 October 2012
Pure and Sn(IV) doped ZnO 1D nanostructures were synthesized by a convenient thermal evaporation method. SEM, EDS, XRD and TEM were used to examine the morphology, composition, phase structure and crystallinity of as-prepared ZnO samples. Raman spectra were used to confirm Sn(IV) doped into ZnO lattice effectively. The Raman spectra of doped ZnO 1D nanostructures also show strong electron– phonon coupling. PL spectra were used to investigate the optical properties of as-prepared samples, which show strong UV emission in pure ZnO while there is dominant green emission in doped ZnO. More interesting, the green emission contains discrete multi-sub-bands because of electron–phonon coupling and optical cavity effect in the doped 1D nanostructures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors ZnO nanostructures Dope Raman Luminescence
1. Introduction ZnO nanostructures are studied widely because of their excellent optical, electrical and photoelectric properties [1]. As well known, doping offers an effective method to adjust the physical properties of semiconductor. ZnO nanostructures were also deliberately doped to modify the electrical/optical properties. For example, Ga–ZnO nanowire arrays showed tunable conductivity and transport properties [2]. The PL band of In–ZnO nanostructures shifted to longer wavelength as comparing with the undoped one [3]. Eu–ZnO nanosheets showed intense red emission due to the high efficient energy transfer from ZnO to Eu3 þ [4]. In the previous study, Sn was considered as one of the most important doping elements to improve the optical properties of ZnO [5]. However, there is no systemic study about how the different dopant concentrations affect on the emission of Sn–ZnO 1D nanosturctures. In this letter, we present a thermal evaporation method to synthesize ZnO 1D nanostructures with different Sn dopant concentration. A detailed examination was carried out on these as-prepared ZnO samples. Different from the reported broad green emission band, the present green emission in doped ZnO 1D nanostructures contains discrete multi-sub-bands.
2. Experimental section Typically, ZnO and C and SnO2 powder with mass ratio of 1: 4: x (x ¼0, 0.1, 0.2, 0.3) was mixed thoroughly and used as a source n
Corresponding authors. Tel./fax: þ86 731 88873055. E-mail addresses: [email protected] (W. Zhou), [email protected] (B. Zou). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.041
material for the synthesis of pure and doped ZnO nanostructures. Si wafer cover with 5 nm Au film was placed downstream to collect product. The furnace was heated to 1000 1C under 50SCCM Ar carrier gas and held at that temperature for 45 min. After the synthesis, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM) were used to examine the as-prepared nanostructures. The room temperature PL and Raman spectra were obtained by using near-field scanning optical microscope (NSOM) (WITec alpha300) with He–Cd laser (325 nm) and Ar þ laser (488 nm) as excitation light source, respectively.
3. Results and discussion Fig. 1a is SEM of pure ZnO nanowires, which deposited on Si substrate randomly. The diameter of pure ZnO nanowires range from 80 nm to 200 nm. EDS (Fig. 1e) demonstrate only Zn and O element (Si come from substrate). Fig. 1b–d are doped ZnO 1D nanostructures with increasing dopant concentration, which tend to wide nanobelts with a width of several micrometer and thickness of tens of nanometer. Fig. 1f–h are corresponding EDS, which show that the dopant concentration increase with the SnO2 in the source material. Fig. 1i is typical low magnification TEM of two crossed doped ZnO nanobelts. Fig. 1j is high resolution TEM (HRTEM) image, which indicates single-crystalline structure of doped ZnO. The lattice stripe spacing in HRTEM is 0.28 nm and 0.52 nm, corresponding to the interplanar distance of (1 0 0) and (0 0 1) as known from bulk ZnO. So, the longitudinal and transverse growth directions of doped ZnO nanobelts are assigned to be [1 0 0] and [0 0 1], respectively. Usually, the growth direction of Zn2SnO4 is o1 1 14, whose lattice stripe spacing is 0.50 nm [6].
370
W. Zhou et al. / Materials Letters 91 (2013) 369–371
50
60
70
Intensity (a.u)
(202)
(200) (112) (201) (004)
(110) (102)
40
$
$ &
30
40
$
0.65% Sn $ $ $ $
&
Au &
30
40
&
50
$ & &
60
Theta (deg)
$
&
50
$ &
$ $ $ $
60
70
&
$
80
Theta (deg)
Intensity (a.u)
Intensity (a.u)
$ $
2.97% Sn
Zn 2SnO 4(24-1470) $
Au
Theta (deg)
$
ZnO (36-1451)
$
$ &
80
0.35% Sn
$ $
70
80
2.97% Sn
$ Au
Intensity (a.u)
30
(103)
0% Sn
(002)
Intensity (a.u)
(100)
(101)
Fig. 1. (a–d) SEM and (e–h) EDS of pure ZnO nanowires, doped ZnO 1D nanostructures with increasing Sn concentration, respectively. (i–j) TEM and HRTEM of doped ZnO nanobelts with 2.97% Sn concentration, respectively.
0.65% Sn
Si
0.35% Sn
$ $
&
30
0% Sn
$ Au &
40
$
50
$
& & & &
60
$
34.4
34.8
35.2
35.6
36.0
36.4
2 Theta (deg) 70
80
Theta (deg)
Fig. 2. (a–d) XRD of ZnO 1D nanostructures with dopant concentration of 0%, 0.35%, 0.65%, 2.97%, respectively. The Au diffraction peaks come from catalyst Au thin film. (e) Enlarged (0 0 2) and (1 0 1) diffraction peaks of the four samples.
In the present HRTEM, the variation of such two lattice stripe spacing (0.52 nm and 0.50 nm) is small and within the measurement error. The Zn2SnO4 may also exist in the ZnO nanobelt matrix. There are some defect regions in the doped nanobelt (see the labeled cycle zone in Fig. 1j) and may also reflect the doped effect. The growth direction of pure ZnO nanowire is [0 0 1] in general. However, the growth direction can change from [0 0 1] when dopant diffuse into ZnO lattice. For example, the growth directions of Ga–ZnO nanowires are [1 0 2] (0.2 at% Ga) and [1 0 1] (1 at% Ga) [2]. Therefore, the present [1 0 0] growth direction may further confirm minor Sn doped into ZnO. XRD patterns of as-prepared pure and doped ZnO are shown in Fig. 2. All diffraction peaks of pure ZnO nanowires (Fig. 2a) are in agreement with the standard values (JCPDS Card No. 36-1451). However, there are several weak peaks in the doped ZnO (Fig. 2b) except for the standard diffraction peaks of ZnO. These weak peaks consist with that of cubic Zn2SnO4. SnO2 decomposes under high temperature and can diffuse efficiently into the lattice of ZnO to form Zn2SnO4. Doping causes a little change to lattice constant,
resulting in measurable higher angle shift in these diffraction ˚ peaks (Fig. 2e) due to the smaller ionic radius of Sn4 þ (0.69 A) ˚ The peaks intensity of Zn2SnO4 increase than that of Zn2 þ (0.74 A). with the percentage of SnO2 in the source material (Fig. 2c and d). Raman spectrum can provide abundant structural information and is powerful for fast and non-destructive detection of dopant. In the Raman spectrum of pure ZnO nanowires (Fig. 3a), the peaks at 334, 383, 440, 593, 991 and 1160 cm 1 can be assigned to E2H-E2L, A1T, E2H, E1L, A1 þE2 and 2E1L modes of ZnO, respectively [7,8]. Fig. 3b is the Raman spectrum of doped nanowires with 0.35% Sn concentration. Besides the typical vibration modes of ZnO, there are two modes at 530 cm 1 and 670 cm 1 with weak intensity, which can be designated to F2g(2) and A1gmodes of Zn2SnO4, respectively[6]. The two vibration modes of Zn2SnO4 indicate that minor Sn doped into ZnO nanostructures successfully. Such result consists with XRD patterns. Another phenomenon worth to note is that peaks at 334, 383, 440, 586, 991 and 1160 cm 1 shift to lower energy (being observation at 332, 381, 437, 584, 985 and 1156 cm 1, respectively) after Sn doping into ZnO. Fig. 3c and d is
400
Intensity (a.u)
381 327
200
800 1000 1200 Shift (cm-1)
434
0.65% Sn
579 667 529
400
1154 980
600
800 1000 1200
Raman Shift (cm-1)
400
600
Raman
800 1000 1200 Shift (cm-1)
433 665
380 325
200
2.97% Sn 1153
527 576
400
978
600
800 1000 1200
Raman Shift (cm-1)
384
0% Sn
490
Intensity (a.u)
1156 985
Intensity (a.u)
381 584 332 530 670
200
600
Raman
(c)
991
0.35% Sn
371
483492 501 0.35% Sn 473 512 467 522 457 534 546 447 558 572 585
387
350 400 450 500 550 600 650
350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
482491 0.65% Sn 500 473 511 465 522 457 533 447 545 558 571 585
Intensity (a.u)
200
1160
586
437
Intensity (a.u)
383 334
0% Sn
Intensity (a.u)
Intensity (a.u)
440
Intensity (a.u)
W. Zhou et al. / Materials Letters 91 (2013) 369–371
483 493 503 2.97% Sn 474 513 466 523 457 534 545 449 557 571 585
350 400 450 500 550 600 650
350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a–d) Raman and (e–h) PL spectra of ZnO 1D nanostructures with dopant concentration of 0%, 0.35%, 0.65%, 2.97%, respectively.
Raman spectra with increasing dopant concentration, 0.65% and 2.97%, respectively. Similar with Fig. 3b, the vibration modes in the two Raman spectra can be designated to E2H-E2L, A1T, E2H, E1L, A1 þE2 and 2E1L modes of ZnO and F2g(2) and A1g modes of Zn2SnO4. However, there are a little difference on the three doped ZnO. The intensity of F2g(2) and A1g modes and full width at half maximum (FWHM) of all modes increase with dopant concentration. Moreover, all the modes shift to short wavenumber with increasing dopant concentration. Such shift come from strong electron–phonon coupling. It is generally accepted that electron– phonon coupling is governed by deformation potential and ¨ Frohlich potential. The present doped system has showed enhanced lattice polarizability and deformation potential. The electron and phonon confinement effect in 1D nanostructures could further improve the coupling intensity. So, the shift, broadening and asymmetry of Raman modes are observed. The electron–phonon coupling intensity can be assessed by intensity ratio of overtone phonon to fundamental phonon [8]. The ratio is 3.5 and 1.1 in the doped and pure nanostructures, demonstrating strong coupling in doped system. Electron–phonon coupling can affect, even dominate the optical properties of nanostructures. The PL spectrum of pure ZnO nanowires (Fig. 3e) contains two bands: one sharp and strong peak and another broad and weak band, which correspond to near-band-gap and defect state emission (such as, oxygen vacancies), respectively [7]. There is drastic difference even little Sn doped into ZnO. The PL spectrum of 0.35% Sn doped ZnO nanostructures is shown in Fig. 3f, which also contains near-band-gap and defect state emission. However, the defect state emission is strong and near-band-gap emission is weak (nearly cannot be observed) in doped ZnO nanostructures. There are lots of oxygen vacancies in doped ZnO because of the competition between Zn and Sn for oxidation. The oxygen vacancies would induce new energy levels in the band gap and trap photo-induced carriers, which result in the dominant green emission. More interesting, the defect state emission contains tens of discrete sub-bands. The average value of energy spacing between neighboring sub-bands (50.5 meV) approach the phonon energy of 440 cm 1 mode. So, the discrete bands may cause by electron–phonon coupling, which also appeared in Raman spectra. The native Fabry–Pe´rot (F–P) cavity effect of 1D nanostructures with flat end facets also play important contribution to the multi-sub-bands. The spacing of modes in F–P cavity is given by Dl ¼ l2/2nL, where L is the optical cavity length, n is the refractive
index and l is the resonant wavelength. The calculated cavity length is about 5–6 mm. Similar with Fe–ZnO microwires [7], the short cavity length indicate that there are multi-cavities within the 1D Sn–ZnO nanostructures. When increasing the dopant concentration to 0.65%, the near-band-gap emission disappear and only defect state emission is observed in the PL spectrum (Fig. 3g). The defect state emission also contains multi-sub-bands. All the sub-band peaks are almost same with that of Fig. 3f. The PL spectrum keep no change under further increasing the dopant concentration (Fig. 3h).
4. Conclusions We synthesized ZnO 1D nanostructures with different dopant concentrations by a simple thermal evaporation method. EDS, XRD, TEM and Raman spectra confirmed that Sn doped into ZnO successfully. The shifted vibration modes in Raman spectra demonstrated strong electron–phonon coupling in doped ZnO nanostructures. The PL spectra show strong UV emission in pure ZnO while dominant green emission in doped ZnO. Moreover, the present strong green emission contains lots of discrete sub-bands. Both electron–phonon coupling and optical cavity effect contribute to the discrete multi-sub-bands. Such emission-modulated 1D ZnO nanostructures can find potential applications in nanophotonics devices.
Acknowledgments The authors thank the NSF of China (Term nos. 51102091, 91121010, 20873039, and 90606001) for financial support. References [1] Fan ZY, Lu JG. J Nanosci Nanotechnol 2005;5:1561–73. [2] Yuan GD, Zhang WJ, Jie JS, Fan X, Tang JX, Shafiq I, et al. Adv Mater 2008;20:168–73. [3] Xu L, Su Y, Chen YQ , Xiao HH, Zhu L, Zhou QT, et al. J Phys Chem B 2006;110:6637–42. [4] Zeng XY, Yuan JL, Wang ZY, Zhang LD. Adv Mater 2007;19:4510–4. [5] Mendoza GA, Trejo CC, Lee J, Bhattacharyya D, Metson J, Evans PJ, et al. J Appl Phys 2006;99:014306–11. [6] Wang JX, Xie SS, Yuan HJ, Yan XQ, Liu DF, Gao Y, et al. Solid State Commun 2004;131:435–40. [7] Li Y, Dai GZ, Zhou CJ, Zhang QL, Wan Q, Fu LM, et al. Nano Res 2010;3:326–38. [8] Wang RP, Xu G, Jin P. Phys Rev B 2004;69:113303–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structure and optical properties of pure and doped ZnO 1D nanostructures Weichang Zhou a,n, Dongsheng Tang a, Ruibin Liu b, Bingsuo Zou b,n a
Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, College of Physics and Information Science, Hunan Normal University, Changsha 410081, China School of MSE, Beijing Institute of Technology, Beijing 100081, China
b
a r t i c l e i n f o
abstract
Article history: Received 12 July 2012 Accepted 9 October 2012 Available online 22 October 2012
Pure and Sn(IV) doped ZnO 1D nanostructures were synthesized by a convenient thermal evaporation method. SEM, EDS, XRD and TEM were used to examine the morphology, composition, phase structure and crystallinity of as-prepared ZnO samples. Raman spectra were used to confirm Sn(IV) doped into ZnO lattice effectively. The Raman spectra of doped ZnO 1D nanostructures also show strong electron– phonon coupling. PL spectra were used to investigate the optical properties of as-prepared samples, which show strong UV emission in pure ZnO while there is dominant green emission in doped ZnO. More interesting, the green emission contains discrete multi-sub-bands because of electron–phonon coupling and optical cavity effect in the doped 1D nanostructures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors ZnO nanostructures Dope Raman Luminescence
1. Introduction ZnO nanostructures are studied widely because of their excellent optical, electrical and photoelectric properties [1]. As well known, doping offers an effective method to adjust the physical properties of semiconductor. ZnO nanostructures were also deliberately doped to modify the electrical/optical properties. For example, Ga–ZnO nanowire arrays showed tunable conductivity and transport properties [2]. The PL band of In–ZnO nanostructures shifted to longer wavelength as comparing with the undoped one [3]. Eu–ZnO nanosheets showed intense red emission due to the high efficient energy transfer from ZnO to Eu3 þ [4]. In the previous study, Sn was considered as one of the most important doping elements to improve the optical properties of ZnO [5]. However, there is no systemic study about how the different dopant concentrations affect on the emission of Sn–ZnO 1D nanosturctures. In this letter, we present a thermal evaporation method to synthesize ZnO 1D nanostructures with different Sn dopant concentration. A detailed examination was carried out on these as-prepared ZnO samples. Different from the reported broad green emission band, the present green emission in doped ZnO 1D nanostructures contains discrete multi-sub-bands.
2. Experimental section Typically, ZnO and C and SnO2 powder with mass ratio of 1: 4: x (x ¼0, 0.1, 0.2, 0.3) was mixed thoroughly and used as a source n
Corresponding authors. Tel./fax: þ86 731 88873055. E-mail addresses: [email protected] (W. Zhou), [email protected] (B. Zou). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.041
material for the synthesis of pure and doped ZnO nanostructures. Si wafer cover with 5 nm Au film was placed downstream to collect product. The furnace was heated to 1000 1C under 50SCCM Ar carrier gas and held at that temperature for 45 min. After the synthesis, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM) were used to examine the as-prepared nanostructures. The room temperature PL and Raman spectra were obtained by using near-field scanning optical microscope (NSOM) (WITec alpha300) with He–Cd laser (325 nm) and Ar þ laser (488 nm) as excitation light source, respectively.
3. Results and discussion Fig. 1a is SEM of pure ZnO nanowires, which deposited on Si substrate randomly. The diameter of pure ZnO nanowires range from 80 nm to 200 nm. EDS (Fig. 1e) demonstrate only Zn and O element (Si come from substrate). Fig. 1b–d are doped ZnO 1D nanostructures with increasing dopant concentration, which tend to wide nanobelts with a width of several micrometer and thickness of tens of nanometer. Fig. 1f–h are corresponding EDS, which show that the dopant concentration increase with the SnO2 in the source material. Fig. 1i is typical low magnification TEM of two crossed doped ZnO nanobelts. Fig. 1j is high resolution TEM (HRTEM) image, which indicates single-crystalline structure of doped ZnO. The lattice stripe spacing in HRTEM is 0.28 nm and 0.52 nm, corresponding to the interplanar distance of (1 0 0) and (0 0 1) as known from bulk ZnO. So, the longitudinal and transverse growth directions of doped ZnO nanobelts are assigned to be [1 0 0] and [0 0 1], respectively. Usually, the growth direction of Zn2SnO4 is o1 1 14, whose lattice stripe spacing is 0.50 nm [6].
370
W. Zhou et al. / Materials Letters 91 (2013) 369–371
50
60
70
Intensity (a.u)
(202)
(200) (112) (201) (004)
(110) (102)
40
$
$ &
30
40
$
0.65% Sn $ $ $ $
&
Au &
30
40
&
50
$ & &
60
Theta (deg)
$
&
50
$ &
$ $ $ $
60
70
&
$
80
Theta (deg)
Intensity (a.u)
Intensity (a.u)
$ $
2.97% Sn
Zn 2SnO 4(24-1470) $
Au
Theta (deg)
$
ZnO (36-1451)
$
$ &
80
0.35% Sn
$ $
70
80
2.97% Sn
$ Au
Intensity (a.u)
30
(103)
0% Sn
(002)
Intensity (a.u)
(100)
(101)
Fig. 1. (a–d) SEM and (e–h) EDS of pure ZnO nanowires, doped ZnO 1D nanostructures with increasing Sn concentration, respectively. (i–j) TEM and HRTEM of doped ZnO nanobelts with 2.97% Sn concentration, respectively.
0.65% Sn
Si
0.35% Sn
$ $
&
30
0% Sn
$ Au &
40
$
50
$
& & & &
60
$
34.4
34.8
35.2
35.6
36.0
36.4
2 Theta (deg) 70
80
Theta (deg)
Fig. 2. (a–d) XRD of ZnO 1D nanostructures with dopant concentration of 0%, 0.35%, 0.65%, 2.97%, respectively. The Au diffraction peaks come from catalyst Au thin film. (e) Enlarged (0 0 2) and (1 0 1) diffraction peaks of the four samples.
In the present HRTEM, the variation of such two lattice stripe spacing (0.52 nm and 0.50 nm) is small and within the measurement error. The Zn2SnO4 may also exist in the ZnO nanobelt matrix. There are some defect regions in the doped nanobelt (see the labeled cycle zone in Fig. 1j) and may also reflect the doped effect. The growth direction of pure ZnO nanowire is [0 0 1] in general. However, the growth direction can change from [0 0 1] when dopant diffuse into ZnO lattice. For example, the growth directions of Ga–ZnO nanowires are [1 0 2] (0.2 at% Ga) and [1 0 1] (1 at% Ga) [2]. Therefore, the present [1 0 0] growth direction may further confirm minor Sn doped into ZnO. XRD patterns of as-prepared pure and doped ZnO are shown in Fig. 2. All diffraction peaks of pure ZnO nanowires (Fig. 2a) are in agreement with the standard values (JCPDS Card No. 36-1451). However, there are several weak peaks in the doped ZnO (Fig. 2b) except for the standard diffraction peaks of ZnO. These weak peaks consist with that of cubic Zn2SnO4. SnO2 decomposes under high temperature and can diffuse efficiently into the lattice of ZnO to form Zn2SnO4. Doping causes a little change to lattice constant,
resulting in measurable higher angle shift in these diffraction ˚ peaks (Fig. 2e) due to the smaller ionic radius of Sn4 þ (0.69 A) ˚ The peaks intensity of Zn2SnO4 increase than that of Zn2 þ (0.74 A). with the percentage of SnO2 in the source material (Fig. 2c and d). Raman spectrum can provide abundant structural information and is powerful for fast and non-destructive detection of dopant. In the Raman spectrum of pure ZnO nanowires (Fig. 3a), the peaks at 334, 383, 440, 593, 991 and 1160 cm 1 can be assigned to E2H-E2L, A1T, E2H, E1L, A1 þE2 and 2E1L modes of ZnO, respectively [7,8]. Fig. 3b is the Raman spectrum of doped nanowires with 0.35% Sn concentration. Besides the typical vibration modes of ZnO, there are two modes at 530 cm 1 and 670 cm 1 with weak intensity, which can be designated to F2g(2) and A1gmodes of Zn2SnO4, respectively[6]. The two vibration modes of Zn2SnO4 indicate that minor Sn doped into ZnO nanostructures successfully. Such result consists with XRD patterns. Another phenomenon worth to note is that peaks at 334, 383, 440, 586, 991 and 1160 cm 1 shift to lower energy (being observation at 332, 381, 437, 584, 985 and 1156 cm 1, respectively) after Sn doping into ZnO. Fig. 3c and d is
400
Intensity (a.u)
381 327
200
800 1000 1200 Shift (cm-1)
434
0.65% Sn
579 667 529
400
1154 980
600
800 1000 1200
Raman Shift (cm-1)
400
600
Raman
800 1000 1200 Shift (cm-1)
433 665
380 325
200
2.97% Sn 1153
527 576
400
978
600
800 1000 1200
Raman Shift (cm-1)
384
0% Sn
490
Intensity (a.u)
1156 985
Intensity (a.u)
381 584 332 530 670
200
600
Raman
(c)
991
0.35% Sn
371
483492 501 0.35% Sn 473 512 467 522 457 534 546 447 558 572 585
387
350 400 450 500 550 600 650
350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
482491 0.65% Sn 500 473 511 465 522 457 533 447 545 558 571 585
Intensity (a.u)
200
1160
586
437
Intensity (a.u)
383 334
0% Sn
Intensity (a.u)
Intensity (a.u)
440
Intensity (a.u)
W. Zhou et al. / Materials Letters 91 (2013) 369–371
483 493 503 2.97% Sn 474 513 466 523 457 534 545 449 557 571 585
350 400 450 500 550 600 650
350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a–d) Raman and (e–h) PL spectra of ZnO 1D nanostructures with dopant concentration of 0%, 0.35%, 0.65%, 2.97%, respectively.
Raman spectra with increasing dopant concentration, 0.65% and 2.97%, respectively. Similar with Fig. 3b, the vibration modes in the two Raman spectra can be designated to E2H-E2L, A1T, E2H, E1L, A1 þE2 and 2E1L modes of ZnO and F2g(2) and A1g modes of Zn2SnO4. However, there are a little difference on the three doped ZnO. The intensity of F2g(2) and A1g modes and full width at half maximum (FWHM) of all modes increase with dopant concentration. Moreover, all the modes shift to short wavenumber with increasing dopant concentration. Such shift come from strong electron–phonon coupling. It is generally accepted that electron– phonon coupling is governed by deformation potential and ¨ Frohlich potential. The present doped system has showed enhanced lattice polarizability and deformation potential. The electron and phonon confinement effect in 1D nanostructures could further improve the coupling intensity. So, the shift, broadening and asymmetry of Raman modes are observed. The electron–phonon coupling intensity can be assessed by intensity ratio of overtone phonon to fundamental phonon [8]. The ratio is 3.5 and 1.1 in the doped and pure nanostructures, demonstrating strong coupling in doped system. Electron–phonon coupling can affect, even dominate the optical properties of nanostructures. The PL spectrum of pure ZnO nanowires (Fig. 3e) contains two bands: one sharp and strong peak and another broad and weak band, which correspond to near-band-gap and defect state emission (such as, oxygen vacancies), respectively [7]. There is drastic difference even little Sn doped into ZnO. The PL spectrum of 0.35% Sn doped ZnO nanostructures is shown in Fig. 3f, which also contains near-band-gap and defect state emission. However, the defect state emission is strong and near-band-gap emission is weak (nearly cannot be observed) in doped ZnO nanostructures. There are lots of oxygen vacancies in doped ZnO because of the competition between Zn and Sn for oxidation. The oxygen vacancies would induce new energy levels in the band gap and trap photo-induced carriers, which result in the dominant green emission. More interesting, the defect state emission contains tens of discrete sub-bands. The average value of energy spacing between neighboring sub-bands (50.5 meV) approach the phonon energy of 440 cm 1 mode. So, the discrete bands may cause by electron–phonon coupling, which also appeared in Raman spectra. The native Fabry–Pe´rot (F–P) cavity effect of 1D nanostructures with flat end facets also play important contribution to the multi-sub-bands. The spacing of modes in F–P cavity is given by Dl ¼ l2/2nL, where L is the optical cavity length, n is the refractive
index and l is the resonant wavelength. The calculated cavity length is about 5–6 mm. Similar with Fe–ZnO microwires [7], the short cavity length indicate that there are multi-cavities within the 1D Sn–ZnO nanostructures. When increasing the dopant concentration to 0.65%, the near-band-gap emission disappear and only defect state emission is observed in the PL spectrum (Fig. 3g). The defect state emission also contains multi-sub-bands. All the sub-band peaks are almost same with that of Fig. 3f. The PL spectrum keep no change under further increasing the dopant concentration (Fig. 3h).
4. Conclusions We synthesized ZnO 1D nanostructures with different dopant concentrations by a simple thermal evaporation method. EDS, XRD, TEM and Raman spectra confirmed that Sn doped into ZnO successfully. The shifted vibration modes in Raman spectra demonstrated strong electron–phonon coupling in doped ZnO nanostructures. The PL spectra show strong UV emission in pure ZnO while dominant green emission in doped ZnO. Moreover, the present strong green emission contains lots of discrete sub-bands. Both electron–phonon coupling and optical cavity effect contribute to the discrete multi-sub-bands. Such emission-modulated 1D ZnO nanostructures can find potential applications in nanophotonics devices.
Acknowledgments The authors thank the NSF of China (Term nos. 51102091, 91121010, 20873039, and 90606001) for financial support. References [1] Fan ZY, Lu JG. J Nanosci Nanotechnol 2005;5:1561–73. [2] Yuan GD, Zhang WJ, Jie JS, Fan X, Tang JX, Shafiq I, et al. Adv Mater 2008;20:168–73. [3] Xu L, Su Y, Chen YQ , Xiao HH, Zhu L, Zhou QT, et al. J Phys Chem B 2006;110:6637–42. [4] Zeng XY, Yuan JL, Wang ZY, Zhang LD. Adv Mater 2007;19:4510–4. [5] Mendoza GA, Trejo CC, Lee J, Bhattacharyya D, Metson J, Evans PJ, et al. J Appl Phys 2006;99:014306–11. [6] Wang JX, Xie SS, Yuan HJ, Yan XQ, Liu DF, Gao Y, et al. Solid State Commun 2004;131:435–40. [7] Li Y, Dai GZ, Zhou CJ, Zhang QL, Wan Q, Fu LM, et al. Nano Res 2010;3:326–38. [8] Wang RP, Xu G, Jin P. Phys Rev B 2004;69:113303–6.