Enhanced Ultraviolet Luminescence of ZnO Nanorods Treated by

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Enhanced Ultraviolet Luminescence of ZnO Nanorods Treated by High-Pressure Water Vapor Annealing (HWA) Amer Al-Nafiey , Brigitte Sieber, Bernard Gelloz, Ahmed Addad, Myriam Moreau, Julien Barjon, Maria Girleanu, Ovidiu Ersen, and Rabah Boukherroub J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09201 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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The Journal of Physical Chemistry

Enhanced Ultraviolet Luminescence of ZnO Nanorods Treated by HighPressure Water Vapor Annealing (HWA) Amer Al-Nafiey1,2, Brigitte Sieber2,*, Bernard Gelloz3, Ahmed Addad2 , Myriam Moreau4, Julien Barjon5, Maria Girleanu6, Ovidiu Ersen6, and Rabah Boukherroub1

1

Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR CNRS 8520), Université Lille 1 Sciences et Technologies, Avenue Poincaré – BP 60069, 59652 Villeneuve d’Ascq Cédex, France.

2

Unité Matériaux Et Transformations (UMET, UMR CNRS 8207), Université Lille 1 Sciences et Technologies, 59655 Villeneuve d’Ascq Cédex, France.

3

Department of Applied Physics, Graduate School of Engineering, Nagoya University, FuroCho, Chikusa-ku, Nagoya, 464-8603 Aichi, Japan 4

LASIR, UMR CNRS 8516, Université Lille 1 Sciences et Technologies, 59655 Villeneuve d’Ascq Cédex, France. 5

Groupe d’Etude de la Matière Condensée (GEMaC, UMR CNRS 8635), Université de Versailles St Quentin, 78035 Versailles Cédex, France.

6

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS UMR CNRS 7504), 23 rue du Loess, BP 43,67034 Strasbourg, France.

*Corresponding author: [email protected]; Tel: +33 320 43 65 94; Fax: +33 320 43 65 91

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ACS Paragon Plus Environment


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Abstract Zinc oxide nanorods (ZnO NRs), synthesized by a low temperature chemical method, were post-annealed at 260°C in air and under high pressure water vapor (HWA) at 1.3 to 3.9 MPa. We found that the UV luminescence intensity increased by a factor 2 to 3 after HWA annealing compared to that observed after annealing in air. Structural analysis of the nanorods in relation with their optical properties by means of Raman and XPS spectroscopies, transmission and scanning electron microscopies allow to conclude that the origin of the UV luminescence enhancement is due to the transformation of the Zn(OH)2 surface layer into ZnO, but also to the growth of a new thin ZnO layer at the surface of the rods. This layer is 12 nm thick and its presence leads to surface reconstruction of the nanorods. In addition, we show that the size and the density of the nanopores within the ZnO NRs are reduced upon HWA annealing with respect to air annealing.

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I. Introduction ZnO is one of the most studied wide band gap semiconductor (Egap = 3.36 eV at 300 K) owing to its outstanding properties, which allow its use for various applications such as biosensors and gas sensors,1 solar cells, transparent thin-film transistors, optoelectronic devices,2, 3 and photocatalysis.4 ZnO nanostructures of different morphologies and sizes can be synthesized by a large variety of solution-based methods at low temperature.3 ZnO nanoparticles,5-7 nanorods,6,

8, 9

nanoplates,8 nanowires,10 nanobelts,11 nanoflowers12 and nanodots13 have already been prepared. In all these ZnO nanostructures, the surface plays a dominant role in their physicochemical properties as a result of their high surface to bulk ratio. Therefore, the luminescence intensity of ZnO NRs is much dependent on the surface quality. ZnO nanostructures can exhibit UV and visible bands. When the visible band emission is produced by an above band gap excitation, it becomes detrimental to the efficiency of the UV band.14 When point defects are present - on the surface or in the bulk - two situations can occur: i) the defects act as radiative recombination centers (RRCs) and a high intensity of the visible band is observed with a small UV to visible intensity ratio, ii) the defects behave as non-radiative recombination centers (NRRCs) resulting in both small UV and visible band intensities even if the UV to visible ratio remains high. The nature of the RRCs at the origin of the visible bands has been controversial: the green band at 2.45-2.5 eV was attributed either to oxygen vacancies V015, 16 or to zinc vacancies VZn;17-19 the yellow band at 2.18 eV was assigned to oxygen interstitial defects Oi20 or to OH groups mostly present in rods prepared by aqueous chemical methods21, 22 or to Zn(OH)2 group bound at the ZnO surface.22, 23 The orange-red emission close to 1.9-2 eV was assigned to clusters of zinc vacancies VZn,15 to interstitial oxygen Oi20,

24, 25

or to V0.26,

27

The red emission at 1.6 eV was attributed to transitions

15

associated with isolated VZn, zinc interstitials Zni28 or interstitial oxygen Oi.29 The defects at the origin of the green band have been localized either in the bulk of the nanorods30 or on their non-polar surfaces.19 NRRCs could be defect complexes involving zinc vacancies VZn (VZn–X complexes)31 or interstitial zinc Zni (Zni–X complexes).32 The defects present on surfaces can also behave like luminescence killers; their capture of electron-hole pairs from the bulk leads to a high surface recombination rate VS, which decreases the bulk outgoing luminescence efficiency. Thus in order to detect a bulk luminescence intensity as high as possible, many attempts have been made to minimize VS and the subsequent intensity losses at the surface. One way to get rid of 3



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point defects located in the bulk and/or at the surface is to post-anneal the ZnO nanostructures. Air, oxygen or forming gas post-anneals are particularly efficient when the nanostructures are elaborated at low temperature since they lead to an enhancement of the luminescence intensity ratio of ultraviolet (UV) to visible emission IUV/IVisible.33-35 Best results are obtained when the anneal temperature is quite high (400-500°C). In the present work we report on the large enhancement of the UV luminescence of ZnO NRs by high water pressure vapor annealing (HWA). HWA has previously been used on SiO236, 37 and on porous silicon.38-42 HWA leads to an increase of the porous silicon photoluminescence and electroluminescence efficiencies and stabilities.38-42 The post anneals of ZnO NRs were performed during 3 h at 260°C in the 1.3 to 3.9 MPa pressure range. For comparison, ZnO NRs were post annealed in air at 260°C for 3 h. Since the ZnO NRs annealed at a pressure of 2.6 MPa exhibited the best UV to visible ratio, a specific attention has been paid to this sample. The as-grown ZnO NRs as well as those post-annealed in air and HWA at 2.6 MPa were characterized by scanning electron microcopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoemission spectroscopy (XPS), micro-Raman, photoluminescence (PL), and cathodoluminescence (CL). A detailed analysis of the results allowed us to conclude that the UV enhancement is surfacerelated. Furthermore, we show that the structural and optical properties of the nanorod ensemble as well as of each nanorod are much more homogeneous after HWA than after air annealing. Since TEM images revealed that both annealing treatments induce a porous structure of the ZnO NRs, electron tomography observations were undertaken. Thus the localization of the pores within the rods as well as the specific influence of HWA annealing on the pore density could be ascertained.

II. Experimental Section A. Materials and Reagents Silicon samples (n-doped, resistivity: 5-10 Ωcm) are provided by Siltronix. Potassium permanganate (KMnO4), tert-butanol, ethanol, zinc nitrate hexahydrate [Zn(NO3)2.6H2O], monoethanolamine (MEA), ammonium hydroxide (NH4OH), acetone and isopropanol were obtained from Sigma-Aldrich.

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B. Preparation of ZnO Nanorods (ZnO NRs) The ZnO NRs were prepared at low temperature (96°C) on silicon substrates via a chemical bath deposition method.5, 6, 9 First, silicon samples were cleaned in successive baths of acetone and isopropyl alcohol in an ultrasonic bath. Before the deposition, a surface pretreatment was performed in PFTE vials filled with 20 mL of deionized water (DW) and 25 mM potassium permanganate (KMnO4) aqueous solution containing 50 µL of tert-butanol. Then, the vials were closed and placed in an oil bath at 84°C for 20 min. The resulting substrates were intensively rinsed with DW and sonicated 10 min in ultrasonic bath. Then, the samples were dipped in 34 mM zinc nitrate aqueous solution (17.5 mL of DW) containing 2 mL of monoethanolamine (MEA) and 500 µL of ammonium hydroxide (NH4OH); the pH is 11.5. The closed vials were put in an oil bath at 96°C for 20 min. After the deposition, the samples were rinsed with DW and dried in air. Then the samples were post-annealed at 260 °C for 3 h either in air or using HWA38, 39 in the 1.3-3.9 MPa pressure range.

C. Characterization of the Nanostructured ZnO Substrates Scanning electron microscopy (SEM) images were obtained using a field emission electron microscope (FESEM) Hitachi S-4700 equipped with a cold field emission emitter and a high efficiency In-lens SE detector. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images, and selected area electron diffraction patterns (SAED) were recorded on an FEI Tecnai G2-20 twin operating at 200 kV. For that purpose, the nanorods were dispersed on a thin carbon film of a 3 mm diameter copper grid. Electron tomography analyses were performed by using a JEOL 2100F S/TEM microscope with a field emission gun operating at 200 kV, a spherical aberration probe corrector and a GATAN Tridiem energy filter. Acquisitions of the tilt images series were performed using a high tilt sample holder from Gatan Company. The irradiation damage was limited by using low electron doses. The angles range from +60 to -70°, with projections taken every 2° according to the Saxton scheme. The acquisition of the 87 TEM projection images was achieved with a cooled CCD detector. The images were first roughly aligned using a crosscorrelation algorithm. A refinement of this initial alignment and a precise determination of the tilt axis direction were obtained in the IMOD software with Au nanoparticles deposited on the TEM membrane used as fiducial markers.43 The volume reconstructions have been computed 5



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using an iterative approach consisting of a simultaneous algebraic reconstruction technique implemented in the TOMO3D software.44 Visualization and quantitative analysis of the final volumes were carried out by using ImageJ software. The 3D model of the structure was obtained in the Slicer3D software. Micro Raman measurements were carried out using a visible Labram HR spectrometer and a UV Labram HR (Horiba). The Raman backscattering was excited with 532 nm excitation wavelength. The beam was focused on the sample surface through an optical objective (x100, 0.9 NA) with a lateral resolution (XY) of less than 1 µm. The spectral resolution is better than 2 cm-1. Overlap of silicon and ZnO signals in the 500-600 cm-1 range was avoided by observing nanorods removed from the silicon substrate and dispersed on the thin carbon film of a 3 mm diameter copper grid. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 220 XL spectrometer from Vacuum Generators featuring a monochromatic Al Kα X-ray source (1486.6 eV) and a spherical energy analyzer operated in the CAE (constant analyzer energy) mode (CAE=100 eV for survey spectra and CAE=40 eV for high-resolution spectra), using the electromagnetic lens mode. The detection angle of the photoelectrons is 30°, as referenced to the sample surface. After subtraction of the Shirley-type background, the corelevel spectra were decomposed into their components with mixed Gaussian-Lorentzian (30:70) shape lines using the CasaXPS software. Quantification calculations were performed using sensitivity factors supplied by PHI. The 325 nm line of a 100 µW HeCd laser was used to record the photoluminescence (PL) spectra at room temperature. The laser beam was focused on a 1 mm diameter spot on the sample. Room temperature cathodoluminescence (CL) images were recorded at 300 K on a Hitachi 4700 FESEM equipped with a Gatan parabolic mirror. A Jobin-Yvon H20 UV monochromator and a Perkin-Elmer photomultiplier were used to record polychromatic CL images. The accelerating voltage of the electron beam was 5 and 15 kV. The beam current was close to 300 pA and the working distance equal to 12.4 mm. The spectral resolution of the CL system was equal to 10 meV. Complementary CL spectra were also recorded at 300K but at lower accelerating voltages, namely 2 and 5 keV. This was done in a JEOL7001F FESEM. The CL emission is collected by a parabolic mirror and focused with mirror optics on the entrance slit of a 55 cm focal length monochromator. The all-mirror optics combined with a suitable choice of UV detectors 6


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and gratings ensures a high spectral sensitivity down to 190 nm. A silicon charge-coupleddisplay (CCD) camera is used to record spectra. The beam current was close to 250 and 580 pA and the working distance equal to 12 mm. The spectral resolution of the CL system was equal to 1.5 meV. Accelerating voltages of 2, 5 and 15 kV correspond to electron penetration depths of 44, 176 and 1060 nm, respectively as simulated by Monte-Carlo with Casino software45 - the value of the penetration depth being estimated as the point where energy losses reach 1% of their maximum. The maximum of energy losses is located at 9.5, 38 and 225 nm from the surface for 2, 5 and 15 kV accelerating voltages, respectively.

III. Results and discussion The ZnO NRs investigated in this work were prepared by a chemical bath deposition method, as reported previously.6, 9 They grow randomly on the silicon substrate and are 100-250 nm in diameter and about 1 µm in length (Fig. 1a). They crystallize in the wurtzite structure and are oriented along the c axis, as shown in Fig. 2 by the HRTEM image and the selected area diffraction pattern (SAED). The spacing of the lattice fringes in Fig. 2 is 0.26 nm, corresponding to the spacing of (002) planes of wurtzite ZnO.

1

a)

2

3

b)

Fig. 1: Morphology of ZnO nanorods synthesized on silicon substrate. a) SEM image at low magnification. The morphology of the as-grown and annealed nanorods looks identical. b) Zoom on nanorods facets. 1: as-grown, 2: annealed in air at 260°C, 3: HWA annealed at 2.6 MPa at 260°C. HWA annealing at 2.6 MPa induces a smoothing of the surface. It is important to notice that the surface of the as-grown ZnO NRs is very rough (Fig. 1b1). Fig. 1b2 and 1b3 depict the SEM images of the ZnO samples after annealing in air at 260°C for 3 h and HWA treatment at 260°C for 3 h, respectively. Surface smoothing took place upon air annealing and even more after HWA treatment. Many structural defects such as dislocations are present in the bulk of the as-grown ZnO NRs, but not in the tips on a length of 150-170 nm (Fig. 2a). After air and HWA annealing, 7



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dislocations disappear giving rise to voids (Fig. 2b and 2c). This point will be discussed later on.

a)

HRTEM

b)

SAED

c)

Fig. 2: a) TEM image of as-grown ZnO NR. The HRTEM and SAED images have been recorded on the tip. The SAED pattern is projected along the [010] zone axis. b) TEM image of ZnO NR annealed in air at 260°C. c) TEM image of HWA-treated ZnO NR (2.6 MPa) at 260°C. To observe more clearly the shape of the rods as well as the localization of the pores with respect to the external surface of the rods, tomographic analyses were performed on the two annealed samples. Representative TEM images extracted from the tilt series used to reconstruct the volumes of the ZnO NRs are shown in Figure 3a and 3b for the two samples (air and HWA). The analysis of the reconstructed volumes of the nanorods in cross-section illustrates a quite hexagonal shape of the nanorods (Fig. 3c and 3d). The presence and the characteristics of the internal porous structure, already observed on the 2D TEM images, are more reliable and a quantification approach can be performed in this regard. We observed that the size of the pores varies as a function of the treatment type. A size between 2 and 16 nm was observed in the case of HWA treatment, while the air annealing provided an increase of the pore diameter ranging from 2 to 30 nm. Moreover, the porous volume was also influenced by the type of treatment, an increase from 3 ± 0.5 % for the HWA annealed sample to 4 ± 0.5 8


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% in the case of the air treated one was observed. Furthermore, the air annealing seems to favor the formation of more small nanopores localized close to the surface of the rod, compared to the ZnO NR annealed under the HWA conditions (Fig. 3e1 and 3e2).

Fig. 3. Electron tomography analysis of representative ZnO NRs annealed in air (a-d) and HWA-treated (e-h). a), e): bright field 2D TEM images extracted from the tilt series at 0° tilt. b), f): typical cross-sectional slices through the analyzed nanorods. e), g) longitudinal slices containing the main axes of the rods. d) and h) 3D models of the reconstructed volumes. The scale bar for the images is 100 nm.

Raman spectra recorded on the reference sample as well as on air- and HWA-annealed ZnO NRs are displayed in Fig. S1a. They consist of several bands which correspond to Ramanactive phonon modes of wurtzite ZnO with a C6V symmetry: E2 (high)–E2 (low) at about 325 cm-1, A1 (TO) at 375-380 cm-1, E1(TO) at 406-423cm-1, E2 (high) at 433–437 cm-1, A1 (LO) between 515 and 550 cm-1 and the E1(LO) phonon mode in the range 564.5-579.3 cm-1. The first mode is allowed and the second one is induced by structural disorder.46 The E2 (low) mode is located close to 93-94 cm-1 (Fig. S1a). The presence of E2 (high) and E2 (low) modes, which correspond to the vibrations of oxygen and zinc atoms, respectively, confirm that the nanorods are made of ZnO, in agreement with TEM results (Fig. 1a). The Raman shifts of the E2 modes are a little smaller than those expected, suggesting the presence of a residual tensile stress in the nanorods certainly due to the difference of the thermal expansion coefficients between silicon and ZnO. The full width at half maximum (FWHM) of E2 (high) mode decreases from 14-15 cm-1 in asgrown nanorods to 9 cm-1 in HWA-annealed ones (Fig. S1b). This last value is still larger 9



than the 6-7 cm-1 expected for undoped ZnO substrates for instance.47 The FWHM of the E2 (low) mode is smaller: 5-6 cm-1 in as-grown nanorods and 4 cm-1 in HWA-annealed rods. This indicates that the disorder in the nanorods is drastically decreased upon HWA-annealing. Asymmetric broadening observed at the lower frequency side (A1(TO) and E1(TO) modes) can be ascribed to disorder in as-grown and air-annealed nanorods. This is confirmed by the presence of the A1 (LO) and E1 (LO) phonon bands, especially in air-annealed nanorods (Fig. S1). These phonon modes are known to be enhanced with increasing disorder sometimes attributed to oxygen vacancies.48 The Raman spectra clearly demonstrate that the nanorods are much more homogeneous after HWA-annealing.

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.35 0.30

HWA - 3.9 MPa

0.25

HWA - 1.3 MPa as-grown

1.4

a)

HWA - 2.6 MPa PL Intensity

PL Intensity

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1.9

air

2.4

Photon Energy / eV

2.9

as-grown

0.20 0.15

air & HWA - 1.3 MPa

0.10

HWA 2.6 MPa

0.05 0.00

3.4

1.4

b)

1.9

2.4

2.9

Photon Energy / eV

Fig. 4: Room temperature PL spectra of the as-grown, air- and HWA-annealed ZnO NRs. a) UV and visible bands b) details of the visible band, λex = 325 nm. When excited at 325 nm, the as-grown nanorods exhibit a PL signal only in the visible part of the spectrum whereas annealing in air induces the presence of a UV band and the decrease of the visible band which results in the increase of the UV/Vis ratio (Fig. 4). The highest value (4.3) of the UV/Vis ratio is very close to the best value (4.4) obtained in ref.49.The absence of a UV band in the as-grown ZnO NRs could result from the presence of a high density of dislocations (Fig. 2a), which can act as non-radiative recombination centers (NRRC) in ZnO.50 But, in such a case, the visible band should be higher in nanorods which do not contain dislocations as in air- and HWA-annealed ones. So, even if dislocations behave as NRRCs, another reason for UV quenching has to be invoked. This will be discussed in more detail later on. The intensity of the visible band remains nearly constant after HWA annealing (Fig. S2b and 5b) along with a large increase of the UV band (Fig. S2a and 5a). The largest integrated UV band intensity is obtained when HWA annealing is performed at 2.6 MPa (Fig. S2a and 5a); at this pressure the UV band intensity is three times larger than that recorded upon air annealing (Fig. 5a). Thus the HWA-treated ZnO sample annealed at a pressure of 2.6 10


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MPa, giving the best UV to visible band intensity ratio (see inset in Fig. 5b), has been studied in more details.

5 air annealed

UV area / Visible area

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4 3 2 1 0 -1

0

1.3

2.6

3.9

HWA pressure / MPa Fig. 5: Variations of the UV to visible band intensity ratio with different treatments, as deduced from PL integrated intensities. It displays a maximum at 2.6 MPa. As shown in Fig. 6a, ZnO NRs annealed in air exhibit a heterogeneous UV luminescence intensity along their length and their tip is more luminescent. This is less the case when the nanorods are annealed under HWA at 2.6 MPa, even if the tips remain more luminescent than the rest of the rods (Fig. 6b). Furthermore, the CL intensity of each nanorod is higher after HWA annealing (Fig. 6 and 7). A variation of the NRRC density could also explain such a phenomenon. The difference in the UV luminescence between air and HWA annealed nanorods increases when their diameter decreases (Fig. 7).

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Fig. 6: Secondary Electrons (SE) and UV-CL images recorded at 15 kV and 300 K on ZnO NRs: a) air-annealed, b) HWA-annealed at 2.6 MPa. The CL images were recorded under the same conditions; this allows the comparison of their intensities directly from the images. Moreover, the difference is more pronounced at 5 kV whatever the nanorod diameter. It can also be seen in Fig. 7 that the tendency curves of air- and HWA-annealed nanorods converge at a lower diameter value at 5 kV (evaluated at 350 nm) than at 15 kV (750 nm). Therefore, the modification of the nanorods surface seems to be more important by HWA than by air annealing, as suggested previously (Fig. 1b).

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170 160

CL Intensity / a.u.

150

HWA annealed 5 kV 15 kV

140 130 120 110 100 90

air annealed 5 kV 15 kV

80 70 50

100

150

200

250

Nanorod diameter/ nm

Fig. 7: 5 and 15 kV CL intensity dependence on diameter of air- and HWA-annealed ZnO NRs. The peak of the UV band is close to 3.293 and 3.3 eV in air- and HWA-annealed nanorods, respectively (Fig. 8). The UV band, whose origin can be ascribed to the free exciton (FX) of ZnO51 has a linewidth close to 120 meV. This is larger than that observed in ZnO thin films having a surface with nanowall-network structures and exhibiting a PL peak position very close to 3.3 eV.51 But it is close to the 131 meV observed in ZnO NRs synthesized by chemical bath deposition (CBD)52 and much smaller than the 160 meV exhibited by the UV band of ZnO NRs also synthesized by CBD.53 The asymmetrical shape is attributed to the presence of FX phonons replicas.54,55 0.6

1.6

air

PL Intensity / a.u.

PL Intensity / a.u.

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0.4

0.2

0.0

0.8 0.4 0.0

3

a)

HWA - 2.6 MPa 1.2

3.2

3.4

b)

Photon Energy / eV

3

3.2

3.4

Photon Energy / eV

Fig. 8: Details of the PL UV band recorded at 300 K on a) air-annealed and b) HWAannealed samples. λex = 325 nm.

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When excited by a 2 or 5 kV electron beam (Fig. S3), the CL spectra of both samples red shift by about 39-40 meV (Fig. S4) as a result of photon recycling (see Supplementary Information).56, 57 As shown in Fig. S6 the UV band intensity increases much more with HWA pressure under 325 nm excitation than under 2 kV excitation. So the influence of the HWA annealing varies from the extreme surface (depth zero) towards the bulk and it seems to be located at a depth of about 50 nm under the surface of the nanorods (Table S1) since there is no increase of the UV band under a 5 kV excitation. This could mean a presence of a thin part of the rods located at the extreme surface which is different from the bulk. Fig. S2b shows that the visible band area is decreased by a factor 6 after air- and HWA- annealing. All the point defects involved in the different red, orange-red, yellow and green transitions (VZn, VO, Oi, Zni) should be concerned by this global reduction This is certainly one reason of the disappearance of dislocations and of the formation of nanopores (Fig. 2 b and 2 c), which occur at a post-annealing temperature lower than those usually used i.e. in argon atmosphere at 400°C58 or at 700°C.59 In HWA-annealed ZnO NRs there are many nanopores of 3-8 nm, less nanopores of 10-20 nm and very few larger ones (about 50 nm diameter) as imaged by TEM (Fig. 2c). In air-annealed nanorods the mean size of the pores is close to 10-25 nm (Fig. 2b). The more precise evaluation of the nanopore sizes and densities by electron tomography shows that HWA annealing leads to a decrease of the pore size and density as compared to air annealing. From our point of view, the creation of nanopores and the disappearance of dislocations by air- and HWA-annealing at 260°C have one common origin, namely the presence of a high density of point defects in as-grown nanorods, which is reflected by an intense visible band. This shows that many different point defects (VZn, VO, Oi, Zni) are present in the bulk. But, in our samples synthesized from low temperature aqueous solution, the expulsion of water vapor during post annealing also accounts for the formation of zinc and oxygen vacancy pairs.60 During ZnO NRs synthesis, hydroxide ions supplied by the aqueous solution are incorporated in the nanorods by substituting on oxygen sites. Then, heating at 260°C removes hydroxide ions from oxygen site, leading to oxygen vacancies which combine with existing zinc vacancies to form nanopores.60 Since HWA anneal supplies water vapor during nanorods heating, less hydroxide ions are removed from oxygen site than under air anneal. A lower density of oxygen vacancies could thus explain the smaller nanopore size and density of HWA annealed ZnO NRs. It can also be supposed that (VZn-X) complexes are formed on the 14


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surface of HWA annealed samples where X holds for either H, H2 or H2O. Their identification, which requires a much detailed Raman characterization coupled with Fourier transform infrared spectroscopy61 is beyond the scope of this paper. Nevertheless, it can already be suggested that (VZn-H2) complexes probably do not form during HWA annealing since, in our experiments, the decrease of the green band (at 2.2 eV) which can result from their formation62 is observed in both HWA- and air-annealed nanorods. The formation of (Vzn--H2O) complexes in HWA-annealed nanorods seems very likely, since their presence leads to a decrease of the green band.63 A specific study of the vapor pressure influence on the green band and UV intensities would allow the determination of the best conditions of HWA-annealing – temperature and time - which lead to the highest UV to visible band intensity ratio. We have observed that its decrease at 3.9 MPa (Fig. 5) is mainly due to a larger green band intensity; between 2.6 and 3.9 MPa its increase is greater than is the decrease of the UV band (inset of Fig. S2b). As shown by Bera and Basak63, the increase of the green band could result from a decrease of the absorption capability of water molecules by the nanorods surfaces. The point defects, more specifically VZn and VO, can also partly induce the disappearance of dislocations in the following way: first, it is known that dislocations are highly mobile in ZnO.59, 64 Thus, during the air- and HWA- annealing they glide in the different available crystallographic planes, 65 and a certain density of them leave the nanorods by the surface. But, that the dislocations glide or not, they can also absorb point defects, namely VZn and/or VO, and then start climbing.

66,67

Since zinc vacancies VZn become mobile at about 260°C,

68

we suggest that the activation energy for dislocation climb should compare fairly well with the activation energy for VZn diffusion, as already found in alumina for Al or oxygen ions.

69

At 260°C, the oxygen vacancies VO and VO2+ are still immobile but they can be combined to mobile VZn to produce nanopores (Fig. 2). The absorption of vacancies by dislocations together with the formation of nanopores leads to a large decrease of the visible band, as seen in Fig. S2b When a dislocation moves outside of a nanorod at 260°C, it leaves a step on the nanorod surface which becomes more defective. The surface recombination velocity Vs should therefore increase, or at least remain constant compared to its value in as-grown nanorods, leaving the total luminescence intensity either smaller or unchanged. This is not what happens in air- and HWA-annealed ZnO NRs which both exhibit a much larger UV band than the as-grown ones. Since it would be more accurate to say that the UV band is quenched in as-grown nanorods, it is difficult to explain its appearance by just a modification 15



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of the surface defects density. Something different should have occurred during the annealing. XPS spectra displayed in Fig. 9 confirm that the nature of the defects located at the surface are not the principal source of the lack of UV band in as-grown samples. It is the nature of the surface itself which is involved since, in as-grown nanorods, it is composed of zinc hydroxide Zn(OH)2 together with stoichiometric ZnO and non-stoichiometric ZnOx. The broadening of the peaks in the XPS spectra could be due to spatial variations along the surface of the relative content of each phase (ZnOH2, ZnOx and ZnO) and of the defect density in each of them. As a matter of fact, any variation of the band bending leads to a modification of the work function, and thus to a variation of the average binding energies as well as to a broadening of the peaks. Furthermore phase fluctuations are certainly present perpendicular to the nanorods surfaces, which enhance the variation of the average binding energies of ZnO and ZnOx as well as the broadening of their peaks. The carbon detected in the XPS spectra could have been introduced during the synthesis of the nanorods and not removed by the anneals performed at a temperature lower than 500°C. 70 The presence of Zn(OH)2, even as a thin shell, has been shown to quench the near band edge (NBE) transition.71 But ZnO and ZnOx are in such proportions (Fig. 9) that it could be expected that a NBE band, even of low intensity, could be detected. Since this is not the case, we suggest that the ZnO layers located at the surface are highly defective; the defects are certainly not only oxygen vacancies but also defects which behave as NRRCs. The high intensity of the visible band can be attributed to point defects in both ZnO and also to Zn(OH)2.71 After air-annealing at 260°C, the ZnO to Zn(OH)2 ratio increases (Fig. 9) and the UV band appears on the luminescence spectrum (Fig. S2a). This shows that Zn(OH)2 has not disappeared completely after a 3 h annealing at 260°C even though it is supposed to be thermally decomposed into ZnO and H2O at around 125°C.72 Growth of Zn(OH)2 and defective ZnOx occurs under air annealing following: ∆

2OH − + Zn 2+ → Zn(OH )2 → ZnOx + H 2O + Zn(OH )2 When annealing is performed at 260°C under HWA pressure, Zn(OH)2 disappears nearly completely and ZnO becomes predominant (Fig 9): +H O Zn(OH )2 ∆ →ZnO 2

16


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It has already been demonstrated that the oxidation of Zn by H2O is stronger than by O273 Zinc hydroxide is slightly soluble in water but the addition of water vapor in a quantity greater than that produced by its thermal decomposition seems to allow the growth of a ZnO layer certainly by reacting directly with zinc vacancies and interstitials, which outdiffuse from

ZnO

160000

SiOx

the bulk.

HWA2.6MPa

140000

Zn(OH)2

ZnOx

100000

ZnO

C=O

120000

80000

air

60000

ZnOx

XPS counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40000

as grown 20000

0

529

530

531

532

533

534

535

Binding Energy / eV

Fig.9: XPS spectra recorded on as-grown, air- and HWA-annealed ZnO NRs. Best fits of the O1s peak. SiOx results from the silicon substrate oxidation by HWA. The structure of the surface of HWA-annealed NRs as observed by HRTEM confirms this assumption. Fig. 10 shows that a new layer of ZnO, which is 1 to 2 nm thick, has grown on the nanorods. XPS technique probes a depth close to 10 nm. So the XPS spectrum of the HWA-annealed sample includes the new ZnO layer and the Zn(OH)2 layer transformed into ZnO. Thus the real characteristics of the 2 nm thick ZnO layer are quite difficult to assess from XPS spectra. Nevertheless the observation of a much larger intensity of the UV band in this sample could mean that it is less defective than the rest of the nanorods. This specific effect of HWA annealing has already been demonstrated on silicon dioxide36, 37 and on porous silicon.38-42 The influence of surface O-related defects on the UV luminescence quenching in as-grown ZnO nanorods has already been well evidenced by aqueous chemical growth experiments in which the amount of dissolved O2 was in-situ modified.49 By simply positionning the substrate closer to the liquid-air interface, a smaller density of surface Orelated defects was detected, and a much larger UV/Vis ratio was observed. All together these

17



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results clearly demonstrate that the optimization of ZnO nanorods UV luminescence definitely requires the minimization of surface O-related defects either during or after growth.

HWA-treated ZnO nanorod

Fig. 10: HRTEM image of a ZnO NR HWA-annealed at 2.6 MPa and 260°C. The arrows point out the new ZnO layer which has grown on the nanorod surface.

Conclusion We have shown that ZnO NRs synthesized by a wet chemical method and annealed under water vapor at a pressure of 2.6 MPa exhibit a much higher UV luminescence band compared with that induced by air annealing at the same temperature (260°C). Analysis of the samples by SEM, micro-Raman, PL, CL, TEM, HRTEM and XPS allowed to conclude that i) air annealing induces a transformation of the initial Zn(OH)2 shell present around the as-grown nanorods into a defective ZnO layer, ii) HWA-annealing induces a less defective ZnO layer, but also generates the growth of a new thin ZnO layer of higher quality at the nanorods surface. The surface of the nanorods is therefore reconstructed. The large decrease of the visible band observed in both air- and HWA-annealed nanorods is believed to be due to the formation of nanopores and to the climb of dislocations by point defect absorption. This could mean that the visible band in the as-grown nanorods is mainly due to the presence of zinc and oxygen vacancies. The water vapor supplied during HWA-annealing decreases the amount of 18


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hydroxide ions which are released from the nanorods. A lower density and a smaller size of the nanopores is thus observed, as compared to those observed in air annealed ZnO NRs. Other oxide materials could also be treated by HWA in order to improve their optical properties. But to our opinion, a new layer could be formed only if the oxide can be grown at low temperature, as it is the case of ZnO, Other oxides would behave as SiO2 for instance in which a decrease of the amount of hydroxyl groups by HWA leads to a less defective layer.

Acknowledgements The authors gratefully acknowledge financial support from the Centre National de la Recherche Scientifique (CNRS), the University Lille 1 and Nord Pas de Calais region. The SEM and TEM facilities in Lille (France) are supported by the Conseil Régional du Nord-Pas de Calais.

Supporting Information: Fig. S1-S7 and Table S1 including Raman spectra, Room temperature CL spectra recorded at 2 kV and 5 kV, UV and Visible PL intensity variations, Normalized PL and CL spectra recorded at 300 K, Monte Carlo simulations of energy loss of 2kV, 5kV and 15 kV incident electrons, Penetration depths of the different excitation beams used in this work, Variation of UV peak intensity with HWA pressure, HRTEM of surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.

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ZnO nanorods after HWA treatment


Enhanced Ultraviolet Luminescence of ZnO Nanorods Treated by

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