Role of Surface Plasmon Decay Mediated Hot Carriers toward the

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Role of Surface Plasmon Decay Mediated Hot Carriers towards the Photoluminescence Tuning of Metal Coated ZnO Nanorods Tejendra Dixit, Iyamperumal A Palani, and Vipul Singh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11526 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Role of Surface Plasmon Decay Mediated Hot Carriers towards the Photoluminescence Tuning of Metal Coated ZnO Nanorods Tejendra Dixit1, I.A. Palani2, 3 and Vipul Singh1, 3* 1

Molecular and Nanoelectronics Research Group (MNRG), Department of Electrical Engineering, IIT Indore, Indore, Madhya Pradesh, India.

2

Mechatronics and Instrumentation Lab, Department of Mechanical Engineering, IIT Indore, Indore, Madhya Pradesh, India. 3

Centre of Material Science and Engineering, IIT Indore, Indore, Madhya Pradesh, India *

Email: [email protected]

ABSTRACT Photoluminescence spectra of metal (Al and Au) coated ZnO nanorods synthesized by a facile low temperature hydrothermal method with in-situ addition of KMnO4 has been investigated. Further, dependence of defect density prior to metal coating on enhancement/suppression of UV and defect related emissions have been investigated. The UV emission from metal coated ZnO nanorods was greatly enhanced whereas the visible emission was significantly suppressed compared with the bare ZnO nanorods. Here, we have proposed a new mechanism elucidating the effect of Al and Au coating, incorporating the fact that non-radiative decay of surface plasmons to hot electrons and hot holes (generated through d-sp interband transitions) can be assigned for UV-emission enhancement and defect-related emission passivation respectively. The recombination of electrons present at defect level of ZnO to the hot holes generated with d-sp transition can be attributed for the suppression of deep level emission rather than the transfer to the Al Fermi level. While, electron transfer from the defect states to the Fermi level and transfer of hot holes to the ZnO valence band level is responsible for UV emission enhancement in Au coated ZnO nanorods. Moreover, we have also discussed the interaction of charge carriers present at various defects states viz. neutral, singly and doubly ionized oxygen vacancies with metals. The observed results were further verified using Kubelka-Munk absorption technique. This work provides a plausible explanation behind the emission tuning of the metal coated ZnO nanorods. Keywords: ZnO nanorods, Hot electrons and holes, Interband transitions, Photoluminescence, Hydrothermal growth, Surface plasmon resonance.

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1. Introduction: Ever since the exploration of ZnO for optoelectronics, suppression of the visible emission (which are actually defect-related emissions) has become one of the biggest challenges in the ZnO based UV light emitting devices especially for ZnO nanostructures grown via solution processed techniques [1]. In the recent past many efforts have been systematically employed to suppress the defect level emissions (DLE) viz., (1) addition of oxidizing agents like KMnO4, K2Cr2O7, H2O2 in the precursor solution during the growth, (2) dielectric coating viz. PMMA, Al2O3, SnO2, MgO, (3) plasma treatment (hydrogen or argon plasma treatment), (4) thermal annealing in the Ar, H2 environment, (5) post growth treatment with H2O2 and most importantly (6) metal coating due to its excellent control over the emission properties [2-14]. Tuning the emission properties of ZnO with metal decoration has attracted the attention of scientific community in the past decade due to its easy control over the emission properties. Metals like Au, Ag, Pt, Al, Ti, Ni and Cu have been reportedly employed towards the systematic tuning of the UV emission and deep level emissions [15-23]. Many theories have been proposed in the literature in order to explain the enhancement/suppression like, transfer of electrons from the defect states to the metal Fermi level combined with surface plasmon resonance and band bending induced charge transfer from metal to the semiconductor etc. [24, 25]. Now, for the case of Au coated ZnO, depending on the work function of the metal the suppression of near band edge emission (NBE) and DLE should be observed as the formation of Schottky contact would definitely decrease the rate of radiative recombination [25]. Contrary to this, enhancement in NBE and suppression in DLE has been reported by many groups [26-28]. Further, metals like Pt, Pd and Cu which generally form Schottky contacts have shown similar behaviour as that of Au [18, 23, 29]. Thus it is clear that, transfer of electrons from defect states to metal Fermi level and coupling of excitons with plasmons can be attributed as a promising reason behind the phenomenon [16, 25]. However, it must be noted here that only neutral and singly ionized oxygen vacancies holds electrons, while doubly ionized oxygen vacancies and oxygen interstitials are electron deficient sites [30]. Although these sites are electron deficient however, complete passivation of DLE has been observed by the researchers in the past [14, 16]. Therefore, it becomes very clear that revisiting the in the above-mentioned mechanism is required in order to completely understand the mechanisms behind passivation of DLE in metal coated ZnO as the mechanism of electron transfer from defect states to Au Fermi level is not sufficient in order to firmly explain the observed phenomenon.


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Now, the transfer of electrons from ZnO defects states to metal Fermi level is reasonably possible for the metals having work function below the level of defect states of ZnO (nearly 4.9 eV). However, in the case of metals like Al which have been reported to form an Ohmic contacts with ZnO, the above-mentioned mechanism cannot completely explain the reason behind the understanding of UV emission enhancement, as the electrons present at defect states encounter significant barrier height and hence such a transfer is likely to be a forbidden transition. Additionally, the detailed mechanism for the UV emission enhancement and passivation of DLE for metals forming Ohmic contacts has rarely been discussed in literature [19, 31]. Therefore it becomes quite necessary to understand this peculiar phenomenon. Now, the plasmonic hot carrier generation due to the interband transitions (d-sp band transitions) in metal nanoparticles are at the forefront of the present research in many areas like photovoltaic, photo detection etc. [32-34]. Reports are available on the hot carrier generation due to the decay of localized surface plasmons at metal semiconductor interface [35]. However, the role of these hot carriers towards the tuning of the emission properties of ZnO nanorods (NRs) has not yet explored. Here, in this article we have presented a plausible mechanism of the emission enhancement/suppression. Plasmonic hot hole and electron generation due to d-sp transition of Al under illumination, as the d band electrons lies ~1.5 eV below the Fermi level of Al play significant role in controlling the emission properties [36, 37]. Hot electrons plays important role for NBE emission enhancement, while recombination of electrons at defect levels with hot holes can be attributed to reduced DL emission. Here, not only the plasmonic transitions but also interband transitions play important role towards NBE enhancement. In addition to this, the transfer of hot holes to the valance band of the ZnO can be assigned as an important contributor for the emission enhancement in the case of Au [34]. Further these transitions have been confirmed by the KM absorption spectra (from 1.5 eV to 2 eV for Al and 2.4 eV for Au) [32, 36, 38]. This work has shed some light on the mechanism behind the effect of metal coating towards the emission properties of ZnO NRs. 2. Experimental Setup: The preparation process includes seed layer coating followed by growth in the precursor solution. In the first step a 2 M colloidal solution of anhydrous zinc acetate, ethanolamine in 2-methoxyethanol was spin coated on the glass substrate. Then the substrate was annealed at 250 °C in air for 5 min to improve the adhesion of ZnO seed layer film on to the substrate [2, 3]. The precursor solutions were prepared by mixing Zn (NO 3)2. 6H2O (0.1 M) with equimolar hexamethylenetetramine. The additive, 0.1 and 5 mM KMnO4 was added to the solution



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immediately prior to the immersion of seeded substrate [2]. The reaction was carried out at 110 °C for 5 h to grow uniformly dense nanostructures. Finally the samples were directly taken out after the reaction without any cooling; later these samples were thoroughly washed with deionized (DI) water and dried. The morphology and crystallinity of the products were observed by a field emission scanning electron microscope (FESEM), Zeiss Supra -55 equipped with energy dispersive spectra and X-ray diffraction (XRD) having Cu Kα source. The PL spectrometer (Dongwoo Optron DM 500 i) having an excitation source consisting of a continuous wave He-Cd laser (excitation wavelength, 325 nm, PMT detector) was used to measure the PL emission from these samples at room temperature. UV-visible absorbance spectra were recorded by (Cary 60 UV-Vis, Agilent Technologies) with a range of wavelengths from 200 nm to 800 nm. The deposition of 10 nm thick Al on these ZnO NRs was subsequently carried out by using rf-magnetron sputtering system. The base vacuum, Ar flow and sputtering power were 5*10-6 mbar, 10 sccm and rf-100 W respectively. The thickness of Al coating was measured using inbuilt crystal detectors. The deposition of 10 nm thick Au on these ZnO NRs was subsequently carried out by using dc sputtering system (Quorem (Q-150 RES)). The thickness of Au coating was measured using inbuilt crystal detectors. 3. Results and Discussion: 3.1 Morphology and Structural Analysis: Figure 1(a, b) show the typical top- view FESEM images of the ZnO NRs synthesized with the in-situ addition of 0.1 and 5 mM of KMnO4 respectively, and hereby named as ZO1 and ZO2 respectively. Vertically oriented NRs have been observed, the detailed mechanism of formation of these NRs has been discussed in our previous reports [2]. The presence of KMnO4 in the precursor solution during NRs growth has been observed to have strong influence on the hydrolysis of hexamethylenetetramine (HMTA), thus more the amount of KMnO 4 more will be hydrolysis rate of HMTA therefore increasing the growth rate [2, 3]. The overall reaction for the growth of ZnO nanorods can be simply understood as follows [2]. (CH2)6N4 + 6H2O ↔ (CH2)6N4-4H+ + 4OH-

(1)

(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3 → NH3 + H2O ↔ NH4

(2)

Zn2+ + 2OH- ↔ Zn (OH)2 ↔ 2H+ + ZnO2 2 - ↔ ZnO + H2O

(3)

MnO4- + 2H2O+ 3e- ↔ MnO2 + 4OH-

(4)


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4MnO4- + 4 OH- ↔ 4MnO42- + 2H2O + O2

(5)

The presence of O2 in the solution will reduce the defects and thus oriented and defect free ZnO NRs can be readily synthesized [2, 4]. The detailed mechanism of ZnO growth with the addition of KMnO 4 has been discussed in our previous works [2]. In short, temporary OH-¨¨¨OH- interactions are primarily responsible for proton transfer reaction, which gives rise to dramatic structural rearrangements [39]. The presence of O2 and additional OH- ions due to KMnO4 hydroxylation can definitely affect the proton transfer rate and thereby the overall reaction rate. In addition to this, formation of powdery growth is generally reported during hydrothermal synthesis of ZnO. However, due to the addition of KMnO4 in the solution, the formation of (CH2)6N4-4H+ ions has ceased, thereby prohibiting the formation of nanodumbbells of ZnO. Therefore KMnO 4 provides an additional advantage of uniform and non-powdered growth of ZnO NRs on glass substrate. The XRD patterns of the as prepared ZnO NRs are shown in the inset of Fig. 1. From the XRD data as a whole, it was confirmed that all diffraction peaks were indexed to ZnO with wurtzite structure (space group: p63mc (180): a=3.2512 Ǻ and c=5.2109 Ǻ) and were in good agreement with the JCPDS file of ZnO (JCPDS 01-0897102). No other peaks were detected within the detection limit of the XRD instrument. The intense and sharp peaks demonstrate that the product is well crystallized [2]. EDS spectra (shown in Fig. 1 (c, d)) clearly revealed presence of Au and Al in the ZnO matrix respectively. 3.2 Optical Properties: Optical absorption spectra of pristine and Al coated samples are shown in Fig. 2. For ZO1 and ZO2 samples as can be observed from the figure, one strong and one small shoulder band at the UV region are recognised from the spectrum. These two bands can be assigned to two separate excitonic characters of A- and B-excitons of the ZnO NRs [40]. Such a clear splitting in the excitonic bands normally only appears in the NRs with low defect density. In NRs with low-crystalline quality and high defect density, these peaks are broad and will overlap each other forming a single broad absorption band in this region [2]. Additionally, a broad band ranging from 485 to 650 nm in the visible region with peaks mainly at 534 and 600 nm have been observed, which are responsible for green, orange and red emissions in ZnO NRs [40, 41]. Moreover, a tail absorption in the NIR region has been observed, the origin of which is still under investigation. These results are actually different from those normally obtained in most ZnO films, in which no absorption band appears in this region. These absorption



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peaks can be assigned to several physical processes in the NRs such as singlet excitation in ionised Oxygen vacancy, zinc interstitial or antisite Oxygen [42]. It has already been mentioned that KMnO4 addition increases the content of Oxygen in the precursor solution and thereby reduction in the defect density has been observed [2]. The effect of which can be clearly seen in the visible region absorption of ZO2 samples, which shows absorption band mainly in the green region. Additionally, the increase in the UV absorption value for ZO2 samples in comparison with ZO1 samples clearly shows the improvement in the optical properties with the reduction of the defects near the band edge i.e. Urbach tail [43, 44]. Now, with Al coating the absorption edges have been observed to be red shifted by more than 20 nm. The red shift and broadening indicated energy transfer between Al and ZnO NRs [31]. The absorption band ranging from 550-800 nm is the characteristic TE and TM plasmon absorption of Al (with dominating TE mode) [36]. The optical absorption of Al constitutes Drude (intraband transition due to electron-phonon collisions) and two interband absorptions [45]. A reasonably strong interband transition in Al is localized in a narrow energy range around ~1.5 eV and can be attributed to the transitions between a pair of parallel bands around the Σ axis on the Г-K-W-X plane [46]. The pronounced interband transition can therefore be assigned to the transfer of oscillator strength from the intraband to the interband modes. Electron-electron interactions are primarily responsible for the spectral weight shift [47]. In Al, interband transitions are expected for all frequencies as w→0, there is no threshold frequency for the onset of interband transitions [46]. The interband absorption in Al includes both the parallel and normal interband transitions arising from the d-sp band transitions [48]. Moreover, for visible and NIR radiation interband transitions are much larger than the intraband transitions hence the dominating mechanism is interband transition only due to the multi-electron effect [49]. The major contributing part of the interband transition is parallel band transition consisting of relatively abrupt absorption edges at

| |

followed by high energy tails which go over into the Butcher interband absorption [50]. Here Vk denotes the Fourier coefficient of the lattice pseudo-potential for reciprocal lattice vector k. Now in this narrow energy interval around interband transition at 1.5 eV, where stronger damping (and thus decreased LSPR de-phasing times) can be expected through interband transitions to electron-hole pair generation [51]. The broad absorption band ranging from 550 to 800 nm can be assigned to the interaction between ZnO defect states and interband transitions of Al and for both ZO1 and ZO2 samples interaction between defect states and interband transitions has been observed to be very effective. Interestingly a small dimple band transition nearly at 1.95 eV has been reported for Al, which arises from the W3 to W1 band transitions with the increased p and s components. Therefore the interaction between ZnO defect states and these two bands can be a possible reason for such a


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broadening in the visible region absorption [50]. Additionally, thermal and lattice disorder as well as surface scattering can also contribute to the broadening of absorption band. The absorption band in the range of 550-800 clearly supports the interband transitions around 1.5 -2 eV which thereby produces hot holes in the d-bands while hot electrons in the sp bands [52]. To investigate the effect of Au coating on the absorption properties, the KM absorption spectra of pristine and Au coated ZnO NRs were performed; the observed results are shown in Fig. 3. The spectrum for ZO1 and ZO2 samples show an absorption edge at 350 nm that is blue shifted with respect to the bulk absorption edge which generally appears at 376 nm, this shift may be ascribed to the nano size effect of the synthesized ZnO [53]. In case of Au coated ZnO NRs, spectroscopy shows one band in UV region at 373 nm and two bands in the visible region ranging from 425 to 690 nm with peaks at 460 and 565 nm respectively [34]. The band position at 373 nm corresponds to the presence of ZnO whereas the other band represents the formation of Au nanoparticles and their coupling with ZnO defect state assisted absorption band. The shift (more than 25 nm) in ZnO band position in the case of ZnO/Au nanoparticles can be attributed to the high electronegativity of Au [34, 54]. As a result, gold withdraws electron density more towards itself, which finally affect the movement of the band position of ZnO. Additionally broadening of the absorption band in the UV region has been observed with metal coating signifying the coupling between excitonic transitions of ZnO and transverse plasmon resonance of Au [54]. More precisely for ZO1 samples which have shown significant defect density, the broadening of the visible region absorption clearly shows the strong coupling between these two states, while for ZO2 samples broadening was observed mainly in the blue and green region of the visible spectra with a diminishing tail in the NIR region arising from the increased contribution of the intraband transition [32]. The interband transitions can be assigned to the transition between d and sp band of Au. The sp band resembles the free-electron dispersion and crosses the Fermi level. Transitions within sp bands are intraband excitations forming the Drude part of the absorption [32].

The threshold photon energies lie in between ~2.1 to 2.4 eV. This produces hot holes in the d band of Au while hot electrons with energies nearly at Fermi energy level [32]. Therefore, the presence of UV and visible absorbance maxima for ZnO/Au clearly demonstrates that the material becomes photoactive in both UV and visible light region [32, 55].



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To examine the optical properties of the pristine and the metal coated ZnO NRs in detail, the PL spectra measurements were performed as has been shown in Fig. 4 (a, b). A sharp band edge emission in near UV region at 385 nm and a wide broad band extending laterally between 450 nm and up to 700 nm with peaks centred around 580 nm were clearly observed in the emission spectra of pristine ZnO NRs as shown in Fig. 4 (a). The sharp emission peak centred 385 nm can be assigned to the NBE of ZnO, while the broad peak centred at 580 nm is related to defect assisted emission of ZnO [2]. It is extensively reported that broad visible emission in the PL spectra of ZnO NRs corresponds to defect-level emission. The origin of these defects, although not fully understood, seems to involve surface defects and oxygen interstitials and oxygen vacancies [2]. Moreover, blue and green emissions can be assigned to the transition of the electrons present at the neutral and singly ionized oxygen vacancies to valence band respectively, while transition from conduction band to oxygen interstitials and doubly ionized oxygen vacancies for yellow and orange emissions [56]. In our previous report we have systematically tuned the defect level emission with the incorporation of KMnO4 in the precursor solution during the hydrothermal growth of ZnO. Moreover, ZnO NRs grown in presence of KMnO4 show significant improvement in near band emission centered at 385 nm along with suppression of defect assisted emissions [2]. Interestingly for ZO2 samples drastic suppression of DLE specifically for orange and yellow emissions has been observed, which can be attributed to the increased oxygen content in the precursor solution. Moreover, visible emission peak was observed to be blue shifted from 589 nm to 572 nm for ZO2 sample [2]. The observed results are in well accordance with our previous observation in absorption spectroscopy. A blue shift (~3 nm) in the NBE peak position has been observed for ZO2 in comparison with ZO1, which can be assigned to the so called Burstein-Moss effect and to that of reduced Urbach tail [14, 57]. Moreover, reduction in the FWHM value for NBE peak from 17.8 to 13.3 nm was also observed with the increase of the KMnO 4 concentration which therefore, suggests the reduction in the recombination channel [58]. The schematic of the above mentioned mechanism is shown in the Fig. 5. The defect states responsible for orange emissions are mostly located near the surface of the NRs and the reduction of the depletion layer width is one of the prime reasons behind the enhanced NBE and reduced orange and red emissions [56]. Furthermore, Al coating has shown significant enhancement in NBE for all the samples. Moreover, significant suppression in DLE has been observed with Al coating. Generally, the presence of defect states in ZnO NRs prior to metal coating plays a significant role in NBE enhancement/suppression, however in the case of Al coating the enhancement factor was observed to be relatively invariant for the samples having different I NBE to IDLE ratios (intensity ratio of the peaks corresponding to NBE emission to that of DLE). Enhancement factor was


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nearly 4.1 and 3.5 for ZO1 and ZO2 samples respectively [4, 25]. Moreover, after coating Al, reduced DLE with peak position mainly in the orange region has been observed. The DLE peak positions were observed at 596 and 592 nm for Al coated ZO1 and ZO2 samples respectively indicating strong red shift from the peak positions of the pristine samples. This red shift of the DLE peak position clearly indicates that Vo+ and Vox states were largely passivized, while Al coating does not play significant role for the passivation of defect states belonging to the orange emission. Now, for Au coating we have observed strong NBE emission and the enhancement factor was observed to be dependent on the defect density prior to the metal coating as shown in Fig. 4 (b) [14, 25]. The enhancement factor was calculated to be 7.9 and 4.2 for ZO1 and ZO2 samples respectively. Contrary to the case of Al coating, in case of Au coating we have observed complete suppression of DLE to the noise limit of the PMT detector. In general, the suppression of the DLE has been attributed to the transfer of electrons from defect states to the metal Fermi level and coupling between excitons and surface plasmons [14, 25]. It must however be noted that, this mechanism seems satisfactory for Au coating only, as the Fermi level is at -5.2 eV i.e. below Vox and Vo+ defect states, thereby the transfer of electrons to the Au Fermi level can be assigned as the possible reason behind the observed suppression of DLE. Although this mechanism is applicable only for defect states having electrons, while for the Vo++ and Oi defect states (capture holes), trapping of holes with Au coating can definitely cause passivation of these states. Now, it is interesting to note here that Al forms Ohmic contacts with ZnO, hence the ZnO conduction band level (4.35 eV) will lie below the Al Fermi level (4.3 eV), thereby theoretically transferring the electrons from Al Fermi level to ZnO CB and defect levels would thus result in the enhancement of the peak corresponding to both NBE and DLE [24]. The schematic of the above mechanism has been shown in Fig. 6 (a, b). Contrary to this, Al decoration has shown NBE enhancement and DLE passivation, indicating requirement of deeper analysis. Therefore in case of Al decorated ZnO NRs the NBE enhancement and DLE passivation cannot simply be assigned to the transfer of electrons present at defect states to Al Fermi level as the gap between the Fermi level and the defect states is sufficient enough to forbid the transfer of electrons [19]. Hence it can be stated that the NBE enhancement and DLE passivation mechanism is different for Al and Au and in this manuscript we have proposed a plausible mechanism to clarify some of the ambiguities. Utilization of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, towards the improvement of the emission and absorption properties can be a suitable solution [32]. Illumination of metallic nanoparticles produces strong optical near-fields that initiate a cascade of processes with multiple outcomes, including the excitation of surface plasmons, their radiative decay to photons and non radiative decay



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into hot carriers [32, 59]. These carriers are considered as hot as these have significantly large energy than those of thermal excitations at ambient temperatures [59]. Further, these carriers can be injected into semiconductor as Landau damping of the collective oscillation of electrons results into highly energetic carriers [32]. This nonradiative decay of surface plasmons at metal semiconductor interface into generation of hot carriers can occur through either direct or phonon assisted transitions [32, 60]. The lowest order process for the non-radiative decay of surface plasmons is the direct generation of electron-hole pair with net zero crystal momentum. This process is allowed above the interband threshold energy [32]. In case of Au the direct transition occur from the d-band to the unoccupied state above the Fermi level, which results in hot holes, which are 2.4 eV more energetic than the hot electrons, while in case of Al, electrons and holes have relatively flat distribution with small fraction of resistive loss [37, 61]. Moreover, upon generation plasmons undergo fast de-phasing with the creation of electron-hole pairs. Plasmon de-phasing comes from collisions with phonons and defects and with electron-electron scattering. The decay of the plasmons can be described as [32]:

Where, nplasmon is the number of plasmons created in the nanoparticle, σ abs is the absorption cross section and I0 is the light flux, generally the coherent lifetime is ~10 fs. Moreover as the fluorescence quantum yields of these plasmons is very low, therefore the probability of radiative decay will be very low [36]. It is reported that plasmon resonance in the d-band electrons also takes place in Al as has also been mentioned earlier in the absorption spectra [36]. The energy of d band electrons lies 1.5-2 eV below the Fermi level of Al in a form of continuous energy distribution [61]. Thus, suface plasmon decay will generate hot holes in the d-bands while hot electrons in sp bands [51, 62]. This hot interband electron-hole pair generation can lead to the hot electron injection into the CB of ZnO NRs thus will enhance the NBE emission. Interestingly the hot holes can easily recombine with the electrons present at the Vox, Vo+ states, thereby reducing the DLE corresponding to the blue and green region. As the hot hole levels are actually below to that of ZnO defect levels, therefore providing energetically favourable path for recombination of these defects level electrons with hot holes causing suppression of defect related emissions in the blue and green region of the visible emission. In addition to this, hot electrons can directly go to the ZnO conduction band level which could lead to NBE enhancement along with the plasmonic transitions. It is interesting to note here that although for ZO2 samples the DLE peak was blue shifted in comparison to that of ZO1 sample, however after Al coating the DLE peak position was mainly in the orange emission as these are related to the Vo++ and Oi defect states [56]. Additionally, the improved


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radiative recombination rate and reduced non-radiative recombination with the increase of conduction band electron density will therefore suppress the orange and yellow emission. Thus it can be stated that hot electrons can play major role in NBE enhancement while hot holes in DLE passivation. As the contributors for the NBE enhancement are hot electrons and plasmonic effects, therefore the enhancement ratio is almost independent of defect density prior to Al coating. Had it been dependent on the defect density, the enhancement factor would not have different for ZO1 and ZO2 samples. Due to the presence of additional transition near K point contributing predominately to hot holes with energies >2 eV and lead to the moderate asymmetry between the electrons and hole energy distribution making hot holes more energetic than hot electrons thereby promoting the recombination between electrons present at defect levels and hot holes. Apart from plasmonic effects, formation of AlOx can cause some passivation as it might change the dielectric environment but the extant will not be high, therefore the enhancement in NBE and defect passivation solely due to AlO x formation can be discarded [19]. Moreover, the blue-shift of about 2 nm from the UV emission of the Al coated ZnO NRs can be surprisingly observed compared with that of the ZnO NRs. Herein, it is believed that the exciton and plasmon interactions are also responsible for the blue-shift of UV emission in the Al coated ZnO NRs [14]. Additional roles may also be played by the phonon subsystem and the harmonic electron–phonon interaction [63]. Hence the blue shift and increased NBE signifies the involvement of surface plasmon resonance. Moreover the enhancement factor invariance over the defect density of the pristine samples supports the abovementioned mechanism. The schematic of the above mechanism has been shown in Fig. 7 (a). Now, metals like Au where the metal Fermi level lie below the defect level of ZnO, theoretically should form Schottky barrier which will further suppress the NBE and DLE [24]. Interestingly in the case of Au also, the enhancement in NBE and suppression of DLE has been observed. Here, the electrons from defect levels can transfer to the metal Fermi level and thus can cause the NBE enhancement. While in case of Al the only possibility for ZnO defect level electrons is to recombine with hot holes. Moreover, hot electrons cannot cross the barrier (due to Schottky nature of ZnO and Au contact with barrier height ranging from 0.65 to 1.1 eV), as these are not energetic and have energies near the Fermi level [32, 34]. In general, the injection of these hot electrons depends on the boundary conditions at the interface between the metal and the semiconductor [32]. Now the condition for injection of these hot electrons is as follows [32]: EF+ω ≥ E ≥ EF + ΔEB (1)

(2)



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For crossing the barrier, above mentioned conditions should be satisfied and the momentum of electron should be in the range of injection cone [60]. The maximum energy of the hot electrons can go upto 0.57 eV, therefore there will be very low probability for these electrons for barrier transition [60]. Now it can be stated that the contributors in the NBE enhancement are the plasmonic transitions, intraband transitions (low contribution in this case) and negligible interband transitions, which are shown in the Fig. 7 (b). Furthermore the enhancement factor in the case of Au coated ZnO NRs were observed to be dependent on the defect density present in the samples prior to the Au coating (7.9 for ZO1 and 4.2 for ZO2), which is in contrast to the case of Al coating where the enhancement factor was almost independent on the defect density present in the samples before metal coating. These observations clearly depict that the mechanism regarding the transfer of electrons from VoX and Vo+ states to the Au Fermi level is the possible reason behind DLE suppression. Moreover the hole present at the Vo++ and Oi states can recombine with hot electrons generated through interband transitions which will lead to the suppression of the orange and yellow emissions by recombining trapped holes present at these sites. Interestingly; in the case of Au coating, the transfer of hot holes to the ZnO VB is favourable as the hot holes lies ~2.4 eV below the Fermi level [34]. It can significantly reduce the depletion layer width as these hot holes can help in desorption of attached O2- ; thus further improving the enhancement in NBE. Additionally it has been observed that Au coating has changed the recombination mechanism from excitonic recombination to electron hole bimolecular recombination. In addition to this the blue shift in the NBE peak has been observed with Au coating indicating involvement of Burstein-Moss effect [14]. Unlike Al, where both hot holes and hot electrons contribute towards NBE and DLE enhancement/suppression, only hot holes play important role in the NBE enhancement while hot electrons have to tunnel or require thermal boost to overcome the barrier and instead of this they play important role in the suppression of defects related to Vo++ and Oi states. Moreover optical band gap of nanostructures have been investigated by UV-visible diffusive reflectance spectra, as shown in the inset of Fig. 8 (a, b). Furthermore, the direct band gap energies (Eg) of the ZnO crystals were calculated by the Kubelka–Munk (KM) method. This method is based on the transformation of diffuse reflectance measurements to estimate the Eg values with good accuracy. The band gap of ZO1 and ZO2 samples were 3.23 and 3.21 eV respectively. Correspondingly, the ZnO nanorods exhibited a lower optical energy gap with respect to bulk ZnO. The band gap variation can be attributed to the variation in size, surface defects and crystallinity. Now, after metal coating drastic decrease in the band gap value has been observed for Al coated samples i.e. 3.02 and 2.89 eV for AlZO1 and AlZO2 samples respectively. Similar trends have been observed for Au coated samples. Most importantly, for Au coated samples hump in the visible region along with the


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Urbach tail can be clearly seen which can be assigned to the interband transitions [64]. Moreover for Al coated samples the continuous energy distribution in addition to the Urbach tail has been observed. 4. Conclusions: Conclusively, the NBE emission and DL emission of ZnO NRs are enhanced and suppressed, respectively, by metal coating, which can be attributed to a combination of hot electron transfer from Al-sp band to ZnO conduction band and recombination of electrons at defect levels with hot holes present at d band of Al, while transfer of electrons present at defect levels to the metal Fermi level in the case of Au. Moreover, charge recombination can be assigned for suppression of the defect emissions corresponding to the Vox and Vo+ assisted emissions in the case of Al. Interband transitions present in the metal have been shown to have strong influence on the emission properties. KM absorption spectra have confirmed the interband transitions nearly at 1.5 eV for Al and 2.4 eV for Au. We further believe that the generation of hot electrons and holes will play a significant role towards the development of highly efficient photoelectric devices. Acknowledgements: One of the authors T. D. is grateful to FESEM and PL facilities equipped at the Sophisticated Instrument Centre, IIT Indore. T. D. would further like to thank the Ministry of Human Resource and Development (MHRD), India for providing the Teaching Assistantship (TA). Authors would further like to acknowledge Dr. Pankaj R Sagdeo and Dr. M. Anbarasu for allowing the usage of UV Visible DRS facility and rf-magnetron facility respectively. References: [1] Lai, C. W.; An, J.; Ong, H. C. Surface-plasmon-mediated emission from metal-capped ZnO thin films. Appl. Phy. Lett. 2005, 86, 251105. [2] Dixit, T.; Bilgaiyan, A.; Palani, I.A.; Nakamura, D.; Okada, T.; Singh, V. Influence of potassium permanganate on the anisotropic growth and enhanced UV emission of ZnO nanostructures using hydrothermal process for optoelectronic applications. J. Sol Gel Sci. Technol. 2015, 75, 693. [3] Dixit, T.; Palani, I.A.; Singh, V. Investigation on the influence of dichromate ion on the ZnO nanodumbbells and ZnCr2O4 nano-walls. J. Mater Sci. Mater Electron. 2015, 26, 821. [4] Tang Q.; Zhou W.; Shen J.; Zhang W.; Kongb L.; Qian Y. A template-free aqueous route to ZnO nanorod arrays with high optical property. Chem. Comm. 2004, 712. [5] Richters, J. P.; Voss, T.; Wischmeier, L.; Rückmann, I.; Gutowski, J. Influence of polymer coating on the low-temperature photoluminescence properties of ZnO nanowires. Appl. Phy. Lett., 2008, 92, 011103.



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u, K.; odr guez-C rdoba,

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Figure Captions: Fig. 1 (a, b) FESEM images of the ZO1 and ZO2 samples; (c) EDS spectra for Au coated ZnO NRs; (d) EDS spectra for Al coated ZnO NRs. The inset shows XRD plots for the samples. Fig. 2 KM absorbance spectra of the pristine and Al coated ZnO NRs. Fig. 3 KM absorbance spectra of the pristine and Al coated ZnO NRs. Fig. 4 PL spectra of pristine and metal coated ZnO NRs (a) with Al coating; (b) with Au coating. The inset shows the emission spectra of the samples in the visible region. Fig. 5 Sketch of the energy band of the ZnO nanorods with two different types of surface depletion region i.e. for ZO1 and ZO2 samples. Fig. 6 Schematic diagram for the mechanism of the emissions in pristine and metal coated ZnO NRs without considering the involvement of hot carriers. Fig. 7 Schematic diagram for the proposed mechanism of the emissions in pristine and metal coated ZnO NRs by considering the involvement of hot carriers. Fig. 8 The [F(R)* hʋ] 2 - hʋ curve for different samples.

TOC Graphic


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Figure 1 85x49mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Role of Surface Plasmon Decay Mediated Hot Carriers toward the

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