This research article provides a pathway of controlled growth of ZnO nano-rods, -flowers, -needles or -tubes without external chemical catalysis, via a simple wet chemical method by control of synthesis temperature. Morphological effects on structural and optical properties are studied by Ultraviolet-visible (UV-vis) spectroscopy shows slight enhancement in the band gap, with increasing synthesis temperature. Photoluminescence (PL) data indicates the existence of defect in the nanomaterials, which is more elaborately explained by schematic band diagram. A sharp and strong peak in Raman spectroscopy is observed at ∼438cm−1 is assigned to the E2high optical mode of the ZnO, indicating the wurtzite hexagonal phase with high crystallinity.

Controlling the morphology of semiconducting nanostructures is the most important parameter for optical-electronic technological applications.1 ZnO, with band gap of 3.3 eV has a wurtzite crystal structure and high exciton binding energy of 60 meV.2,3 It has received lot of attention as a nanostructured material due to its unique properties related to numerous applications, such as in opto-electronic devices (light emitting diodes (LEDs), laser diodes, solar cells, photodetectors), energy harvesting devices (Nano generators), electronic devices (transistors), sensors, catalysts, piezoelectric etc.4 Despite rising evidence of significant Eco toxicity it is often regarded as non-toxic and biocompatible,5 its synthesis does not require toxic precursors.

A number of studies on the preparation technique of ZnO nanostructure are reported using wet chemical, electro-deposition, template assisted route, thermal evaporation, chemical vapor deposition (CVD) techniques, most of these techniques require sophisticated instruments to generate high temperature or high pressure. Wet chemical is the most simple and cost effective technique to prepare highly crystalline ZnO. To further ensure economically cheap technique, we have used amorphous glass substrates for the synthesis of nano structure. But the true challenge lies in the synthesis conditions which control the crystal growth in a particular fashion to grow desired nanostructures moreover prevention of cluster formation. Previously, this complex growth mechanism is mostly explained based on catalyst-driven mechanisms (solution-liquid solid6,7 and vapor-solid-solid8 growth) and vapor-liquid-solid (VLS)9,10 mechanism. But recent studies show that axial screw dislocation mechanism drives the spontaneous formation of nano-wire/tube structure.11,12 Size and morphology plays a vital role in the physical properties of the nanomaterial. So here, we have focused on the variation of optical properties with the change in shape and size of ZnO nano structures.

The objective of this report is to control morphology of ZnO nanostructure to tune structural and optical properties by controlling synthesis temperature in the wet chemical method. Here, growth of ZnO nanostructures in the form of nano-flowers, nano-needles, nano-rods and nano-tubes is demonstrated just by controlling the synthesis temperature with excellent crystallinity. UV-vis spectroscopy, photoluminescence and Raman spectroscopy measurements are performed to analyze the optical properties of the synthesized structures.

Zinc Acetate dehydrate (Zn (CH3COO)2. 2H2O) (100mM: Alfa Aesar chemicals) is used as the precursor for preparation of pure ZnO films on glass substrate by the most simple and inexpensive hydrothermal method on a large area at atmospheric pressure. An aqueous solution of Zinc Acetate is prepared in a 100 ml double distilled water as starting material. After complete dissolution of chemicals in distilled water, ammonia is added drop wise to form resultant solution with a pH∼13. The substrates are cleaned first with distilled water, followed by Acetone and Ethanol by ultra-sonication for 10 min in each solution and air dried. Glass substrate is kept vertically in the solution. To understand the growth mechanism more clearly, reaction time (120 min) was kept constant and reaction temperature is varied from 70 °C to 120 °C. Deposited films are taken out from the solution and washed with distilled water for several times to remove over deposited/ ammonia related impurities from the film. Dried films are annealed in air at 150 °C for 2 hrs to obtain pure phase ZnO.

The phase purity, morphology and composition of ZnO films are investigated by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer)with Cu-Kα radiation(λ = 1.54Å) and field emission scanning electron microscope (FESEM, Supra 55 Zeiss). Optical band gap is determined with the Diffuse Reflectance Spectrometer (Agilent Cary-60 UV-Vis). RT fluorescence spectroscopic measurements is conducted using spectrofluorometer (Horiba Jobin Yvon, Fluorolog-3) having Xe lamp source with an excitation wavelength of 325 nm. Micro Raman Scattering measurement of the samples is recorded using Labram-HR 800 spectrometer equipped with excitation radiation at wavelength of 488 nm from an argon ion laser at a spectral resolution of about 1 cm−1. The nomenclature for the samples prepared and studied in this manuscript are as mentioned in Table I.

TABLE I.

Deposition temperature and sample name assignment.

Deposition temperature (°C) Sample name (Deposition Time 120 min)
70  ZN1 
80  ZN2 
90  ZN3 
100  ZN4 
110  ZN5 
120  ZN6 
Deposition temperature (°C) Sample name (Deposition Time 120 min)
70  ZN1 
80  ZN2 
90  ZN3 
100  ZN4 
110  ZN5 
120  ZN6 

Figure 1 shows the XRD pattern of ZnO nano-structure deposited on glass substrate at different deposition temperatures keeping deposition time constant (120 min.). All major peaks (2 ⊖ ∼ 30° to 60°) can be assigned to the pure hexagonal phase of wurtzite-type ZnO structure with space group P63mc. Fig 1(a) shows that at such a low synthesis temperature a highly crystalline and pure phase of ZnO has been obtained. The film grown below 120°C shows a more preferred orientation along (002) planes in comparison to powder samples where (101) planes have the strongest peak intensity.13 However the nano-material films grown at 120 °C, shows a reduced relative intensity of the (002) reflection, disappearance of (00l) plane is might be due to the change in structure.14 Fig 2(a) shows Reitveld refinement plot of ZnO deposited at 90 °C using “Fullprof” software. Refined lattice parameter value of the nano structures are shown in Fig. 2(b).

FIG. 1.

(a) XRD pattern of (a) ZN1 (b) ZN2 (c) ZN3 (d) ZN4 (e) ZN5 (f) ZN6 nanostructures and (b) relative shift in (002) peak position.

FIG. 1.

(a) XRD pattern of (a) ZN1 (b) ZN2 (c) ZN3 (d) ZN4 (e) ZN5 (f) ZN6 nanostructures and (b) relative shift in (002) peak position.

Close modal
FIG. 2.

(a) Reitveld refinement of ZnO deposited at 90°C with deposition time of 120 min. (b)The effect of deposition temperature on lattice parameter of ZnO.

FIG. 2.

(a) Reitveld refinement of ZnO deposited at 90°C with deposition time of 120 min. (b)The effect of deposition temperature on lattice parameter of ZnO.

Close modal

Fig 1(b) shows (002) peaks shift towards the higher diffraction angle until growth temperature of 110 °C (ZN1 to ZN5) while for growth at 120 °C (ZN6) the peak shifts to the lower diffraction angle. This trend further verified by Reitveld refinement data, lattice constant decreases until growth temperature 110 °C followed by increase for 120 °C (Fig. 2(b)). There might be two possible reasons for the nonsystematic change in lattice parameter with growth temperature; (1) due to the change in the morphology of ZnO nano structure (2) due to internal compressive micro strain. Effect of strain developed in the nano structure grown on glass substrates were analyzed using Williamson-Hall15 formula (Table II). Comparing Fig 2(b) and Table II, it is clear that decrease in lattice parameter has strong correlation with increase in strain in the nanostructures or vice versa.

TABLE II.

Shows the value of crystalline size and strain (ϵ) for the ZnO nano structures grown at different deposition temperature obtained by two Debye Scherre’s method and Williamson-Hall method.

Debye Scherre’s method Williamson- Hall method Uniform Deformation Model (UDM)
Sample D (nm) D (nm) Strain (ϵ) x 10−3
ZN1  33.21  84.51  1.53 
ZN2  37.32  73.72  0.97 
ZN3  36.72  107.44  1.72 
ZN4  36.93  197.18  2.18 
ZN5  36.03  206.86  2.33 
ZN6  35.80  55.66  1.57 
Debye Scherre’s method Williamson- Hall method Uniform Deformation Model (UDM)
Sample D (nm) D (nm) Strain (ϵ) x 10−3
ZN1  33.21  84.51  1.53 
ZN2  37.32  73.72  0.97 
ZN3  36.72  107.44  1.72 
ZN4  36.93  197.18  2.18 
ZN5  36.03  206.86  2.33 
ZN6  35.80  55.66  1.57 

Here we have calculated crystalline size and induced strain of the nano structures using Debye-Scherrer’s formula (Eq.(1)) and Williamson and Hall using uniform deformation model (UDM) (Eq.(2)). Expressed as

D = K λ β D cos θ
(1)

and

β hkl = K λ D cos θ + 4 ε tan θ
(2)

where D = crystalline size, K = shape factor (0.9), λ = wavelength of Cu radiation and β is full width half maxima of a corresponding peak and ε is induced strain value. All the deduced parameters are listed in Table II. From Table II it can be seen that stain is the order of 10−3, which is strongly dependent on the crystallite size. To elucidate, higher the crystallite size is, higher is the strain and vice versa.

Fig. 3 shows FESEM images of ZnO films (magnification at 5 KX) deposited at different temperatures (70- 120 °C) and inset of the figure shows magnified image of the same sample (magnification at 80 KX).

FIG. 3.

FESEM images ZnO nanostructures (a) ZN1 (b) ZN2 (c) ZN3 (d) ZN4 (e) ZN5 (f) ZN6 for deposition time of 120 min.

FIG. 3.

FESEM images ZnO nanostructures (a) ZN1 (b) ZN2 (c) ZN3 (d) ZN4 (e) ZN5 (f) ZN6 for deposition time of 120 min.

Close modal

The morphology and structure of ZnO nanostructure is not solely dependent on the preparation technique. Other external conditions such as pH value (H+ ion concentration), reaction temperature, reaction time, solution concentration, etc. also plays a vital role in hydrothermal method.16 Comparing the nano-structure size (nano-rods’ diameter) from Fig. 3, it is easily noticeable that nanorods formed at 70 °C (Fig 3(a)) are bigger than the structures at 90 °C (Fig 3(b)). Due to the fast nucleation process at higher temperature, bigger rods are divided/branched into smaller rods, with a decrease of size from 323 nm (ZN1) to 206 nm (ZN3) respectively. Afterwards, due to the disk shape layer formation, the structure size again increases. The average diameters of rods samples from ZN1 to ZN6 are 323 nm, 210 nm, 206 nm, 228 nm, 271nm and 339 nm, respectively.

For a better understanding, schematic (Fig. 4) of idealized and proposed formation mechanism of ZnO nanostructure is drawn based on the observed nano structure in FESEM. ZnO consists of an interesting structure with both metastable polar and stable non-polar, face.17 Zinc-terminate (001) and oxygen-terminate (00 1 ¯ ) are two polar faces (along c-axis) while the other non-polar faces are (010) and (110) (parallel to c-axis) so the growth along the (001) direction is fastest with respect to other faces (Fig. 4(a)).18 Relation velocity crystal growth along different plane is well studied by Wen-Jun Li et al.,19 i . e . V 001 > V ( 01 1 ¯ ) > V 010 > V 011 > V ( 00 1 ¯ ) . The most plausible mechanism known for ZnO nano wires and tubes are either screw dislocation along the axial direction or layer by layer. From figure 3(a), 3(b), 3(c) and 3(f), it clearly shows that screw dislocation mechanism (as indicated in Fig. 4(b), 4(c)and4(e)) may be most possible as a hexagonal top with screw like structure can be easily realized. In Fig. 3(d) and 3(e), no clear hint of screw dislocation can be found. So in Fig. 3(d) and 3(e), layer by layer mechanism may be responsible (as indicated in Fig. 4(d)) along with screw dislocation and mechanism is under debate. Firstly the nano-niddle like structure appears at low temperature and small deposition time along (00l) direction and form a tapered shape nano-needle on top. With the increase in reaction temperature further, the structure reduces their surface energies and tapered shape changes to flat top and gradually hexagonal rods starts appearing (Fig 3(c), 3(d)and3(e)). It is also seen that there are not many active sites around the ZnO nuclei at low reaction temperature. Therefore, the formed nuclei with limited growth rate can get attached together along the preferential direction to form nano-flower like structure. It is well known that crystal plane, whose growth rate is slow, easily appears and whose growth rate is fast easily disappears. So the (00l) plane easily dissolve with respect to other non-polar face at higher temperature (120 °C) resulting the formation of nano tube (Fig. 4(e)).16 

FIG. 4.

Schematic diagram (a) shows idealized growth process of nano-rod with tapered tip. Schematic diagram of growth of (b) Nano-needle (c) Splitting of nano-rod into small nano-rods (d) layer formation along c-axis (e) Nanotube.

FIG. 4.

Schematic diagram (a) shows idealized growth process of nano-rod with tapered tip. Schematic diagram of growth of (b) Nano-needle (c) Splitting of nano-rod into small nano-rods (d) layer formation along c-axis (e) Nanotube.

Close modal

To check the plausible mechanism of ZnO nanostructures, we performed the experiments at fixed deposition temperatures (at 70 °C and 120 °C) and varied the deposition time at the interval of 30 min and deposition was carried out at 30 min, 60 min, 90 min and 120 min. Fig. 5 shows of FESEM images of films deposited at 70 °C for 30 min -120 min in (a-d) and 120 °C for 30 min -120 min from (e-h), it noticeably visible that the rods and tubes synthesized at different temperature and time evidenced screw dislocation mechanism as reported by Yang et al., Jin et al. and Bierman et al.20–22 Screw dislocation mechanism might responsible for modification of rods into tubes as reported by Yang et al., Jin et al. and Bierman et al.20–22 Also one cannot rule out layer by layer deposition in these rods and tubes as from the side view of the rods and tubes one can obviously realize the layer like structure.23 

FIG. 5.

ZnO at 70 °C with depositing time (a) 30 min (b) 60 min (c) 90 min (d) 120 min and 120 °C with depositing time (e) 30 min (f) 60 min (g) 90 min (h) 120 min.

FIG. 5.

ZnO at 70 °C with depositing time (a) 30 min (b) 60 min (c) 90 min (d) 120 min and 120 °C with depositing time (e) 30 min (f) 60 min (g) 90 min (h) 120 min.

Close modal

The effect of deposition temperature on the intrinsic optical properties of the pure ZnO was studied at room temperature (RT) by UV-Vis spectroscopic. Fig. 6. Shows the UV-visible spectra of ZnO (only shown the data in the range of (350 nm -450 nm). It is visible that the band gap is varying from 3.11 eV to 3.19 eV as the deposition temperature increase. The band gap was determined from Tauc plot (using Kubelka-Munk method):

F ( R ) h ν = A ( h ν - E g ) n

where, F(R∞) is the Kubelka-Munk function, A is a constant, Eg is the band gap value and n is an unit less parameter with a value 2 or 1 2 for indirect or direct band gap semiconductors, respectively.

FIG. 6.

Room temperature UV- Vis reflection spectra of ZnO nanostructures. Inset image shows (a) Tauc plot of Zn1 and (b) Variation in bandgap with synthesis temperature.

FIG. 6.

Room temperature UV- Vis reflection spectra of ZnO nanostructures. Inset image shows (a) Tauc plot of Zn1 and (b) Variation in bandgap with synthesis temperature.

Close modal

Inset (a) of Fig 6 shows a typical example the Tauc plot for the ZN1 sample. From the inset (b) of Fig. 6, showing the variation of band gap with growth temperature, it is very clear that the band gap is smaller (∼3.11 eV) in needles ZN1, intermediate in rods (∼3.14 eV) (ZN2-ZN5) and highest in tubes (∼3.19 eV) ZN6. From band-gap studies it is distinctly visible that the synthesis temperature plays vital role, which tunes the band-gap and helps to decide the device manufacturing parameter, which is easily tunable in hydrothermal method by growth temperature.

Synthesis temperature dependent PL studies reveals comprehensive information about the nature of light emission and the fundamental material properties. Generally two emission bands in the PL spectrum of ZnO is observed, one is in the UV range associated with excitonic recombination or band-edge emission and another is in the visible range, which originates from the electron–hole recombination at a deep level, caused by oxygen vacancy or zinc interstitial defects.24 

Fig. 7 shows room temperature PL emission of the samples over a broad range from 3.3 to 2.1 eV (330 to 590 nm). With increase in growth temperature of ZnO films, NBE peak position shifts toward higher energy side, irrespective of defect which is responsible for PL. The existence of Zn interstitials(Znin), Oxygen vacancies (Vo), Zn vacancies (VZn), Oxygen interstitials (Oin), Antisite oxygen (OZn) have been reported as possible defects in PL previously. Fig. 7 shows the PL spectrum of the nano-materials synthesized at different temperatures from 70-120 °C (ZN1-ZN6). The emission band is composed of a weak UV band around 3.12 nm, a weak blue band around 2.66 nm and a strong orange band around 2.20 nm. The UV emission band must be explained by a near band-edge transition of wide band gap ZnO nanorods, namely the free excitons recombination through an exciton–exciton collision process.25 Similarly, an orange band was also observed and it was attributed to the intrinsic defect in ZnO as oxygen interstitials26,27 suggesting oxygen excessive in the sample. We can conclude that the ZnO nanorod has a strong ability to absorb oxygen to form oxygen interstitials defects on the surface. In the case of the weak blue emission, the exact mechanism is not yet clear.28 It may also relate to the surface defects in the present condition. From Fig. 7 it is very clear that the PL spectra strongly depends on the morphology of the ZnO nano-structures as can be seen that for needles (ZN1) intensity is low at higher energy levels. For rods it’s almost flat with higher intensities, expecting certain emissions and overlap for ZN2-ZN5. For tubes (ZN6) the intensities is higher at higher energy level and then decreases. For ZnO rods (ZN2-ZN5) the yellow and green light emission is prominent from PL spectra while for ZnO nano tubes violet light. It indicates the synthesis temperature is one of the important factors to control the surface morphology which ultimately controls the optical properties. So this research paper presents the important message to the basic and applied scientists that in order to tune the optical properties the control over synthesis temperature in hydrothermal method is very important.

FIG. 7.

Room temperature PL spectra of ZnO grown at different temperatures.

FIG. 7.

Room temperature PL spectra of ZnO grown at different temperatures.

Close modal

Fig. 8(a) shows the typical Photoluminescence spectra of ZnO nanorods grown at 90 °C measured at room temperature (excited at 325 nm) which is deconvoluted into 15 well-resolved peaks. The “■” shows experimental data. Solid green and red lines are Gaussian fitting of individual peaks and sum of all peaks, respectively. Fig. 8(b) shows the schematic band diagram. First peak E1 at 3.14 eV is correspond to UV region and it is related to near band emission present in ZnO films. E2 at 3.05 eV and E3 at 2.92 eV is considered to appear from the swallow donor Zni to valance band (VB) and conduction band to shallow acceptor VZn respectively. Both these transitions responsible for violet-blue emissions. A broad blue peak E4 at 2.82 eV in between conduction band (CB) to Oin level and small peak E5 (2.75 eV) in between Znin and VZn, these both peak are known to originate due to the recombination of electron-hole pair.

FIG. 8.

(a) PL spectra of ZnO deposited at 90° C temperature. The “■” shows experimental data. Solid green and red lines are Gaussian fitting of individual peaks and sum of all peaks, respectively. (b) Schematic band diagram of ZnO nanostructure for ZN3.

FIG. 8.

(a) PL spectra of ZnO deposited at 90° C temperature. The “■” shows experimental data. Solid green and red lines are Gaussian fitting of individual peaks and sum of all peaks, respectively. (b) Schematic band diagram of ZnO nanostructure for ZN3.

Close modal

Another intense blue peak at E6 (2.65 eV) was attributed to electron transition from Znin to Oin acceptor level. The origin of green emission in ZnO is one of the most controversial issue. Liu et al.29 ascribed to Zni and Oi, According to Vanheusden et al.30 it is related to VO. Fig. 8(a), shows three prominent peak E7, E8 and E9 at 2.58, 2.53 and 2.44 eV, respectively. Calculation based on full potential linear muffin-tin orbital method explained that the position of oxygen vacancies (VO) level is located at approximately 2.46 eV below the CB.31 So these three emission are attributed to CB or deep level or trap-state oxygen singly charged VO defect state. We also observed a very strong yellow luminescence peak at 2.20 eV (E13) and orange luminescence at 2.15 eV (E14) below the conduction band, these defects are originated due to antisite oxygen (OZn) defect state and complex of V OZni cluster i.e. combination of two point defect VO and Zni, respectively.32 

The optical phonon properties of the ZnO nanostructures prepared at different temperature have been investigated by Raman spectroscopy, and the results are presented in Figure 9. ZnO has a wurtzite structure that belongs to the space group C46v with two unit formulas per primitive cell, with all atoms occupying C3v sites.33 The group theory predicts the existence of the following optical modes at the Γ point of the Brillouin zone: Γ = A1 + 2B1 + E1 + 2E2. ZnO has 12 branches consists of polar modes (A1 and E1), two non-polar modes (2E2) and two silent (2B1) Raman modes.34,35 Among these, A1 and E1 polar modes are divided into transverse optical (TO) and longitudinal optical (LO) phonons due to the long-range electrostatic forces.33,36 It is well understood that the electrostatic forces dominate the anisotropy in the short-range forces, the TO-LO splitting is larger than the A1-E1 splitting, whereas, E2 is divided into E 2 low and E 2 high active modes. The A1 and E1 branches are both infrared and Raman active, the two non-polar E2 branches are Raman active only, and the B1 branches are inactive. The atoms move parallel and perpendicular to the c axis, for the lattice vibrations with A1 and E1 symmetries, respectively. The low-frequency E2 mode ( E 2 low ) is related with the vibration of the heavy Zn sub-lattice, while the high-frequency E2 mode ( E 2 high ) comprises simply the oxygen atoms. According to the theory , the scarring peaks at ∼ 438, 381 and 583 cm−1, corresponding to the fundamental optical modes of E2, A1(TO), and A1(LO), respectively14 and they have been attributed to the intrinsic defects, such as oxygen vacancy and interstitial zinc.

FIG. 9.

RT Raman spectra of ZnO nanostructures synthesized at different temperature.

FIG. 9.

RT Raman spectra of ZnO nanostructures synthesized at different temperature.

Close modal

The spectrum of all ZnO nano structure grown at different temperature is similar (Fig 9). Table III summarizes the Phonon mode frequencies (in units of cm−1) of wurtzite ZnO nano structures. By comparing the obtained modes of ZnO with literature, one can assign the peak around 98 cm−1 (P1) to E2(low) mode. There is a secondary phonon mode has presented at 150 cm−1, which is assigned as 2E2low.37 The small peak ∼330cm−1 (P3) is assigned to E2high- E2low (multi-phonon process) and is known to be a second order vibration mode arising from zone-boundary phonons.38 Another small peak, P4 is observed at ∼380cm−1 is assigned toA1(TO) mode. P5 around 438.1cm−1 is assigned to E 2 high which the characteristics of wurtzite structure and good crystallinity. Wide peak around 580 cm−1 (P6) is assigned to the combination of A1 and E1 longitudinal optical mode, (LO (A1+ E1) mode. A red shift in P6 can be explained on the basis of possible mechanisms; (1) spatial confinement within the nano-rods/nano-tubes boundaries; (2) Phonon localization by the defects such as oxygen deficiency, zinc excess, surface impurities etc.The broadening of the spectra and strong red shifts of LO modes are reported due to the optical phonon confinement.39 Table III shows that crystalline size of ZnO nanostructures are around 100 nm so phonon confinement might be responsible for the observed P6 peak shift.

TABLE III.

Phonon mode frequencies (in units of cm−1) of wurtzite ZnO films deposited at different temperatures (ZN1-ZN6).

Position of the vibration bands (cm−1)
Peak ZN1 ZN2 ZN3 ZN4 ZN5 ZN6 Phonon mode
P1  98.7  98.7  98.7  98.7  98.7  99.7  E2low 
P2  149.9  149.9  150.5  150.5  150.5  149.9  2E2low 
P3  330.1  333.1  333.1  332.5  329  331.8  E2high−E2low 
P4  380.8  380.2  380.8  381.4  379.6  382.1  A1(TO) 
P5  438.1  438.1  438.1  438.1  438.1  438.1  E2high 
P6  583.3  581.5  580.9  580.3  579.1  579.1  LO(A1 + E1) 
Position of the vibration bands (cm−1)
Peak ZN1 ZN2 ZN3 ZN4 ZN5 ZN6 Phonon mode
P1  98.7  98.7  98.7  98.7  98.7  99.7  E2low 
P2  149.9  149.9  150.5  150.5  150.5  149.9  2E2low 
P3  330.1  333.1  333.1  332.5  329  331.8  E2high−E2low 
P4  380.8  380.2  380.8  381.4  379.6  382.1  A1(TO) 
P5  438.1  438.1  438.1  438.1  438.1  438.1  E2high 
P6  583.3  581.5  580.9  580.3  579.1  579.1  LO(A1 + E1) 

Simple technique for the highly-ordered growth of ZnOnano-flowers, nano-rods and nano-tubes films on glass substrates by controlling merely growth temperature is established. The XRD studies indicate the films are highly c-axis oriented. Reitveld refinement confirms the first decrease in the lattice constant until growth temperature 110 °C and then increase for 120 °C, even though it is small. The FESEM images clearly indicate the change in the ZnO morphology with synthesis temperature, at lower temperature (7 °C) needle flowers, at (80-110 °C) rods-flowers and at (120 °C) tube formations evidenced. The possible growth mechanism of needles, rods and tubes are elucidated. Depending on the morphologies naturally band-gap was found to contrast. PL data indicates the intensities and defect levels changes with synthesis temperatures too. The presence of defects formed in highly non-equilibrium conditions had a significant impact on the luminescence of ZnO. We demonstrated a superior control not only on the morphology but also on the defect levels for nano-rods/tubes by controlling synthesis temperature. Such ample defect level adjustments will greatly benefit the applications of ZnO nano-needles/rods/tubes in light emission, opto-electronic devices, biological labelling, display devices, etc.

This work was supported by the Department of Science and Technology, India by awarding the prestigious ‘Ramanujan Fellowship’ (SR/S2/RJN-121/2012) to the PMS. PMS is thankful to Prof. Pradeep Mathur, Director, IIT Indore, for encouraging the research and providing the necessary facilities. We are thankful to Dr. Vasant Sathe, IUC-DAE Consortium for Scientific Research, Indore for his help to do Raman measurement of the samples. The help received from, Dr. Anjan Chakraborty is also acknowledged. Authors are thankful to SIC Indore for providing the research facilities like XRD and FESEM.

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Supplementary Material