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Band Gap Tuning and Room Temperature Photoluminescence of Physically Self-assembled CuO Nano-column Array 2

Indrani Thakur, Sriparna Chatterjee, and Nilesh Kulkarni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10597 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 25, 2015

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The Journal of Physical Chemistry

Band Gap Tuning and Room Temperature Photoluminescence of Physically Self-Assembled Cu2O Nano-Column Array Indrani Mukherjee,a,b Sriparna Chatterjee,a,* Nilesh A. Kulkarnic a

Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, AcharyaVihar, Bhubaneswar 751013, India b

National Institute of Technology, Durgapur, 713209, India

c

Department of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India

ABSTRACT: The morphology modification of nanostructured materials is of great interest due to controllable and unusual inherent properties in such materials. In this work, growth and morphology evolution of well-separated Cu2O nanocolumn array is explored by tuning deposition angle and substrate rotation speed in oblique angle deposition (OAD). Xray diffraction and photoluminescence measurements reveal that crystal quality and optical properties crucially depend on the morphology of the nanostructures. Intense room temperature photoluminescence (RTPL) signal at 525 nm indicates that the Cu2O nano-columns arrays exhibit strong green luminescence. The morphology dependent bandgap tuning makes this approach of great potential for nanoscale opto-electronic applications.

9,

Introduction One-dimensional nanostructures with unique texture, good adherence to the substrate have attracted considerable attention to the researchers due to their signifi1,2,3,4,5,6 cant contribution in various fields of research. However, these types of 2D or 3D nano-architectures are challenging to manufacture. Other than multi-beam optical interference lithography, which is known to be a promising technique to create two- dimensional (2D) and three- dimensional (3D) nano-structured solids rap7 idly and with limitless structural complexity, oblique angle deposition (OAD) have emerged as a simple way to fabricate uniform 3D nanostructure with unique ma2,Error! Bookmark not defined. It offers a simterial properties. ple, single-step, time efficient method to fabricate nanostructured arrays of various materials with good adherence to substrate. OAD is a physical vapor deposition method where the main growth mechanism is the 8 atom “shadowing effect” which is a “physical selfassembly” process through which obliquely incident atoms/molecules can only deposit to the tops of higher surface points, such as to the tips of a nanostructured array or to the hill-tops of a rough or patterned substrate and this can be used to create 3-D nanostructures. Initially, the roughening of the surface occurs due to the inherently random character of the sputter deposition process, and the subsequent development of the columnar structure is highly dependent on the deposition angle. At oblique deposition angles, topographically elevated points, created randomly, preferentially intercept the incoming flux while shadowing lower regions from

,

,

the incoming adatoms 10 11 12 that creates 3D nanoarchitecture. In this work, we report the growth of wellseparated 3-D nano-architecture of Cu2O via OAD. Various inorganic semiconductors have explored with much attention due to their ease of fabrication with high chemical and thermal stability and tunable electronic properties in the numerous prospective fields of applica13 14 15, , tions like field emission, catalysis, gas sensor, 16 17 op18,19 20 toelectronics and solar cells. Among them Cu2O has gained attention over others due to the following properties: (i) it is one of the first known p-type direct band 21 gap I-VI semiconductor with a band gap of 2·17 eV that enables it to be a promising material for the conversion 22 of solar energy into electrical or chemical energy, (ii) it effectively adsorbs oxygen molecule that scavenge the photo- generated electrons to restrict the recombination of excitons, which plays very important role in improvement of the efficiency in photo catalytic processes 23 and of course (iii) it is low toxic and earth abundant. Synthesis of one-dimensional nanostructures of 24,25,26,27 Cu2O using the techniques like electrodeposi28,29 30 31 tion thermal annealing, stress-induced method etc. have earlier been reported by several groups but these methods may not be used to precisely control the crystal texture of the as-grown material. To the best of our knowledge this is the first work on incident angle dependent, (111) oriented growth of well-separated Cu2O nano-column array by oblique angle sputter depo22 sition. The nano-columns grown by S. Swain et al., were not well-isolated from each other as they have used oblique angle of 45°, which is not very large to create

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dominant shadowing effect during deposition and it should be mentioned here that they have kept the substrate rotation speed constant for their depositions. Here, in this work we have investigated the role of deposition angle as well as the substrate rotation on the morphology and optical, especially band gap tuning and photoluminescence property of physically self assembled nano-columnar structures. We have found that not only deposition angle, but also substrate rotation speed plays important role in the growth of well-isolated nanocolumnar structures that in turn influence the optical property of the material. The as-grown {111}-oriented, well-separated nano-columns are expected to steer their potential application in the fields of catalysis, electrical, photoelectrochemical and opto-electronic devices.

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Highscore Plus Software was used to analyze the XRD data and using Scherrer formula the crystallite sizes were calculated. X- Ray photoelectron spectroscopy was done using an Al- Kα X- ray source (VG Scienta, USA). Specular reflectance, transmittance measurements were done in dual beam configuration in a Cary UV-Vis spectrophotometer. Prior to each optical measurement, baseline correction was done using silver mirror. Steady state photoluminescence measurements were conducted at an excitation wavelength of 473 nm (using ozone free Xe arc lamp of power 450 W) using a FLS980 Combined Steady State and Lifetime Spectrometer, from Edinburg Photonics. Results and discussion Structural and chemical analysis:

Experimental All Cu2O samples were grown both on glass and Si (ntype, As-doped, Ω < 0.005 Ω cm) substrates in a custom made sputter deposition chamber (Excel instruments), equipped with a Glassman DC power supply operated at 40 W in constant current mode (200 mA/ 400 V). The glass substrates were ultrasonically cleaned with surfactant, acetone, alcohol and deionised water successively while the Si substrates were used as received. A 2 inch Cu disc was used as the target material and the chamber -7 was kept at a base pressure of 2x 10 torr and working -2 pressure of 2.6 x 10 torr. The target to substrate distances was 75 mm and three deposition angles (αD = 0º, 45º and 85º) were chosen in order to grow well-separated nano-columnar structures of Cu2O (Figure S1 in Supporting information represents the schematic of the position of gun and substrate holder at normal incidence and oblique incidence of 85º). Three different rotational speeds (Φ) of 1, 5 and 25 rpm were applied for each deposition angle. Henceforth, the samples will be represented as Dα-RΦ, where α is the deposition angle and Φ represents substrate rotation speed. All the depositions were done at room temperature and the deposition time was fixed in such a way so that the thickness of all the films remains nearly constant for the proper comparison of the crystallographic and optical properties. The deposition time of normally deposited films (D0-RΦ) was 30 minutes whereas for obliquely deposited films, the deposition time was increased to 45 minutes for D45-RΦ and 60 minutes for D85-RΦ samples.The deposition rates of the Cu2O thin films were about 56.6 ± 4 nm/min, 40.0 ± 3.5 nm/min and 41.6 ± 4.2 nm/min for the deposition angles of 0°, 45° and 85° respectively, as measured by the analysis of cross- sectional scanning electron microscope (SEM) images. In order to remove any formed oxide layers on the target surface, the Cutarget was pre-sputtered in pure Ar atmosphere for 2 mins. A Ziess Ultra Field Emission Scanning Electron Microscope was used to image both plan view and crosssectional morphological features of the Cu2O thin films. PanAlytical Xpert Pro X-Ray Diffractometer (XRD) in specular diffraction mode with θ-θ geometry along with stationary sample stage and without rotation in ψ direction was used to obtain the crystallinity and phase purity of the as-deposited Cu2O films. PanAlytical Xpert

The crystallographic phase analyses of the as-prepared samples are done with the help of XRD. Figure 1(a) represents the XRD patterns of the as-grown thin films. All films, grown at normal incidence (αD = 0°) (D0-R1, D0R5, D0-R25) shows polycrystalline nature with diffraction peaks at 2θ = 29.9º, 36.8º, 62.0º, which corresponds to cubic cuprite structure (JCPDS Card No. 77-0199). The sample grown at oblique angle of 45° with lowest substrate rotation speed (1 rpm) (D45-R1) is also found to follow similar polycrystalline crystalline nature. But for the same incidence angle (αD = 45°), as the substrate rotation speed is increased to 5 and 25 rpm the films show a tendency to grow along (111) plane of cubic Cu2O. The orientation along (111) plane becomes much prominent in case of the samples that are deposited at highest oblique incidence of 85° (D85-R1, D85-R5 and D85-R25). During sputter deposition process, a –ve potential is applied to the Cu target and the chamber is at ground potential. When Ar is introduced in the chamber to maintain the required working pressure, Ar gets ionized as it encounters a potential difference between target and its surrounding. Then positively charged Ar ions are attracted to the negatively biased gun (or copper target) and as a result the Ar ions transfer their kinetic energy to the copper target and remove or sputter out the Cu atoms from copper disc. When oxygen gas is also mixed with argon gas then sputtered copper atoms react with oxygen and forms copper oxide in vapor (gas) form. These vapors get adhere to the substrates. By varying Ar/O2 ratio, different phases of copper oxide can be achieved. If the substrate is placed in line of sight of the vapor flux (i.e. αD=0°), then copper oxide vapor can directly reach to the substrate making a uniform growth of nanostructures but when the substrates are kept at an angle then incoming flux cannot reach the surface directly. They are partially shadowed by the neighboring taller structures and thus growth rate of the nanostructures are hindered.

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Figure 1. (a) XRD patterns of the as-sputtered Cu2O thin films grown by oblique angle deposition at three different incidence angles with varying substrate rotation speed, and (b) XPS of D85-R25 sample.

presence of uniform granular microstructures of dimension ~ 92 nm. From the top view it seems that the film is very compact in nature. Now, as we increase the angle of incidence from 0° to 45°, the inter-columnar separation increases as seen in samples D45-R1 and D85-R1 (Figure S3c and S3e in supporting information). Figure S3d (in supporting information) represents the top view morphology of D45-R1 which shows presence of clustered tips of the nano-column arrays and the clustering appears to be more prominent as we go to even higher deposition angle of 85° (top view of sample D85-R1 presented in figure S3 (f) in supporting information).

With increase in incident angle the growth rate decreases which leads to texurization. During slow growth condition islands with energetically favorable crystal facets predominate over others. In our study, we also find that samples grown at highest oblique angle (D85-R1 and D85-R25) show a preferential orientation along (111) plane, whereas samples grown at normal incidence do not show any particular crystal orientation. The average crystallite size of all the samples are calculated using the Scherrer formula and found to be within the size range of 9-16 nm which confirms that all films are nanocrystalline in nature.

The compositions of the as-grown samples are analyzed by EDX at different positions throughout the sample. An average Cu: O ratio is found to be 2.6 : 1 (Figure S2 in supporting information). Peaks at 8.9 keV (Cu Lα), 8.0 keV (Cu Kα), 0.8 keV (Cu Lα) and 0.5 keV (O Kα) confirmed the presence of both Cu and O elements in the sample. Contribution of gold is seen as the sample was gold-coated prior to the SEM study. The EDX analysis approximately shows that the samples are oxygen deficient which indicate the presence of Cu2O phase. Therefore, to further clarify the presence of Cu2O phase, XPS was done and figure 1(b) represents the X-ray photoelectron peaks of the copper for sample D85-R25. Two major peaks at 932.8 eV and 952.3 eV are seen which corresponds to the binding energies of 2p1/2 and 2p3/2 states of Cu+1.32 All the above-mentioned structural and chemical compositional analyses indicate that the as grown samples are phase pure Cu2O with no other impurities. Morphological evolution of physically self- assembled Cu2O nano-columns:

Both plan view and cross-sectional scanning electron micrographs of the as-sputtered Cu2O samples grown at different incidence angles (αD =0°, 45° and 85°) with a substrate rotation speed of 1 rpm are represented in figure S3 of supporting information. As evident from the crosssectional view, the thicknesses of all the samples are measured to be in the range of 1.7 to 2.0 μm. For sample D0-R1 (αD =0°, φ = 1), the formation of nano-columnar morphology is observed but hardly there is any separation between two individual columns (Figure (S3a) in supporting information). In figure S3(b)(in supporting information) the plan view morphology of this film shows

Figure 2. (a) Cross-sectional SEM images of Cu2O thin films grown at deposition angles of 0°with substrate rotation speed of 25 rpm (D0-R25), (b) plan view SEM of the same, and (c) Cross-sectional SEM images of Cu2O thin films grown at deposition angles of 85°with substrate rotation speed of 25 rpm (D85-R25); inset shows corresponding plan view morphology.

Earlier the influence of different rotational speeds on the development of Si nanostructures with 33 different morphology was explored by Patzig et al. They found that in glancing angle deposition substrate rotation plays crucial role in the formation of wellseparated one-dimensional nanostructures with different morphology like nanorod, nanospiral etc. In order to check the influence of substrate rotation in making separated nano-columnar structures, we therefore, increased the substrate rotation speed by 5 times. But in the cross-sectional morphology of the samples, D0-R5, D45-R5 and D85-R5, we do not see any major change in the inter-columanar separation (figure not shown). The substrate rotation speed was further increased to 25 rpm while the incident flux angles were kept constant, i.e. αD = 0°, 45° and 85°. Figure 2(a) represents the cross-sectional SEM image of the film designated as D0-R25, which shows the formation of very narrowly spaced columnar structure and the top view SEM image (Figure 2(b)) shows similar



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morphology like D0-R1 (Figure S3(b) in supporting information). When the deposition angle was increased to 45°, then the columns start to separate out and finally at the deposition angle of 85° the desired well separated vertically aligned nano-columns of width 100-130 nm and height 2.5 μm are formed (figure 2(c)). Interestingly, the top view morphology of films D45-R25 and D85-R25 (inset of figure 2(c)) samples are found to be composed of rectangular clusters, but the cluster size (~ 75 nm × 160 nm) of D85-R25 film is smaller than the film D85-R1 (~ 90 nm × 185 nm) and the inter-cluster separation is ~ 40 ± 10 nm. It appears that with simultaneous increase in deposition angle and rotational speed, the films become more porous. It is well established that by controlling substrate rotation, one can induce a morphological shift 33 from rod like to screw like or spiral like structure. A fast rotation of substrate induces a low number of particles per rotation value, which imparts a unidirectional oblique angle flux density to the substrate, which finally leads to the formation of self- assembled rod like structures. In addition to that, a high oblique incidence angle makes the shadowing effect more prominent which results in the larger inter- columnar separation.

Figure 3. High magnification cross-sectional SEM image of (a) D0-R1 and (b) D85-R25 Figure 3 represents the high-resolution cross-sectional scanning electron micrographs of the films D0-R1 and D85-R45 to show the nano- columnar morphology evolution of film D85-R25 from D0-R1. Therefore, the morphology study manifests that the precise control over both deposition angle and substrate rotation are key factors to grow well-separated one- dimensional morphology of Cu2O by oblique angle sputter deposition technique. Optical properties and tailoring of band gap of Cu2O nanocolumns:

In order to investigate the effect of light scattered from the surface of thin film, the as-grown samples are characterised using UV-Vis NIR spectroscopy at room temperature. In figure S4 (a and b) in supporting information, reflectance data of the samples, which are grown at normal incidence (0°) and highest oblique incidence (85°) are represented. In both cases it is observed that with increase in substrate rotation speed, the reflectance value decreases (by approximately 2-3%). Earlier such lowering in reflectance is attributed to the porous nature of the film by Chatterjee et al.34 Therefore, in order to calculate the percentage porosity of the as- grown films, refractive indices (R.I) of all the samples were calculated using the following equation22, in the thin film.

n

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(1)

Figure S5(a) and (b) represents the plot of refractive index as a function of wavelength. Refractive indices are found to share an inverse relationship with substrate rotation speed for all samples.Percent porosity of the films is calculated using the refractive indices obtained at 550 nm. The representative equation22,34,35 is as follows: Porosity 1

100%

(2)

Where, nd is the refractive index of pore free Cu2O (2.5) and n is the refractive index of the as-prepared Cu2O thin films at 550 nm. For films that are deposited at normal incidence with substrate rotation speed of 1 rpm, the calculated porosity is ~ 54%. With increase in substrate rotation the porosity increases to ~ 60%. The films deposited at oblique angle of 85° with lowest substrate rotation speed of 1 rpm, shows comparable percent porosity of 61%. But films with isolated nano-columnar morphologywere found to show at least 25% increase in porosity. Films deposited at 45° with different substrate rotation speed show percent porosity in the range 5760%. The summary of change of porosity and refractive indices with deposition parameters and crystal structure is represented in table 1 of supporting information. The drastic change in porosity for the sample D85-R25 compared to all other thin films is in line with the morphological evolutions as revealed in SEM study.

A plot showing correlation between porosity and refractive index as a function of rotational speed for two different set of samples (i.e. αD = 0° and 85°) reveals that with increasing deposition angle refractive indices are lowered and porosity of the thin films are increased. We see the substantial effect of substrate rotation on enhancing the porosity for obliquely deposited films (αD = 85°) compared to the films deposited at normal incidence (figure 4(a)). Figure S6 represents the transmittance spectra of the sample D0-R25, D45-R25 D85-R25. It is seen that the (111) oriented nano-columnar thin film, D85-R25, is higher transmitting than polycrystalline D45-R25 and D0R25, which is in agreement with the previous observations by Chandra et al.36

Figure 4. (a) Plot showing relation of refractive index (R.I) and porosity of the thin films as a function of deposition angle and (b) Tauc plot of the Cu2O thin films sputter deposited at 85° with varying deposition angle.

Generally, the optical transmittance is known to be influenced by several factors like porosity, growth orientation


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and crystallite size37 etc. The (111) oriented nano-columnar thin film shows lower refractive index (1.3) compared to its other counterparts. Therefore, orientation-dependent refraction at the grain boundaries will be less, which in turn will minimize the scattering at the grain-boundaries and thus transmittance will be enhanced. Figure 4(b) represents the optical band gap of the samples deposited at three different deposition angles with a substrate rotation of 25 rpm. The optical band gap is calculated using the following equation:38 αhυ Chυ E

(3)

where, α is the optical absorbance coefficient, T is transmittance, R is reflectance and d is thickness of the film. α can be calculated by the equation,

α ln !

#

(4)

The as-calculated band gap for D85-R25 thin film sample is highest (2.44 eV), the band gap for D45-R25 and D0R25 are calculated to be 2.22 eV and 2.02 eV respectively. This kind of blue shift in absorbance spectra has been reported to occur with a decrease in particle size. But for our system the crystallite sizes are almost in the comparable range. Therefore, this blue shift cannot be related with quantum size effects.39 Therefore, this blue shift in optical band gap seems to be influenced by carrier concentration.40 In the well-separated nano-columnar structures with high anisotropic crystal orientation, the loss of charge carriers due to trapping at grain boundaries is expected to be minimized and thus charge carrier concentration of D85-R25 sample will be more than D45-R25 and D0-R25 which do not show any preferential crystal orientation. Therefore the blue shift in the band gap of D85R25 sample may be discussed in the light of Burstein41,42 Moss effect which states that an increase in the carrier concentration blocks the lowest states in the conduction band thereby causing blue shift in the optical band gap value. Photoluminescence of Cu2O nanocolumns: Photoluminescence (PL) study is a potential tool to know about the fate of excitons and transitions associated with them. In PL, two types of phenomena are found to occur: (i) band- band transition, (ii) exciton transition. Band to band transition gives information about the separation condition of photo-generated charge carriers. Figure S7 in supporting information shows PL spectra of the samples D0-R25, D45-R25 and D85-R25. All the samples show one emission band in the green region that corresponds to the band-edge transition of Cu2O. Unlike the blue shift in the absorption band-

edge (figure 4(b)) in the absorption spectra, here we do not see any clear blue shift. Rather all the spectra are broad in nature. The higher PL intensity of D85R25 film compared to D45-R25 and D0-R25 can again be ascribed to the presence of higher carrier concentration. In figure 5(a), the change in PL intensity of

the samples that are grown at a particular deposition angle (85º) with different substrate rotation speed is compared. This plot indicates that with increasing substrate rotation, well separated one- dimensional morphology gradually evolves that in turn increases the effective surface area of the sample which contributes in the enhancement of PL intensity, as earlier discussed by Yang et al.43 To understand the charge transfer behavior, the broad emission peak of D85R25 in the green region is resolved into different subemission peaks (Figure 5(b)). The peak at 509 nm can be assigned to the radiative recombination of the photo-generated electrons, which agrees well with the absorption spectra (Figure 4(b)). The other three peaks at 519.6 nm, 537.2 nm and 566 nm can be attributed to the defect arising from copper ion vacancies.44,45 A acceptor level is known to be created at 0.08 eV above the valence band and hence a transition from the conduction band to this acceptor level gives rise to an emission with energy 2.38 eV (519 nm). The donor level is known to be placed at 0.13 eV below the conduction band. Therefore, the emission at 537 nm (2.31 eV) is attributed to the transition between the donor level to the valance band. The broad emission band at 566 nm (2.19 eV) corresponds to the transition from the donor level to the acceptor level of Cu2O.

Figure 5. (a) Steady state room temperature photoluminescence (PL) spectra of the Cu2O thin films, (b) PL spectra of D85-R25 is fitted into several subbands.

Therefore, the optical property of the Cu2O nano-columns suggests that it could be a suitable material for potential barrier layers in Cu2O based devices as we can tune the band gap of the material only by controlling the morphology. This material can also be used as visible light emitting material in order to fabricate photo-electronic devices. In addition to that, the higher transmittance of nano-columnar thin film sample in the visible region makes it important from optical application aspect. Conclusions In summary, for the first time, we have reported the growth of {111}-oriented, well-separated Cu2O nanocolumns by oblique angle sputter deposition. We have thoroughly investigated the role of deposition angle and substrate rotation speed in the formation of physically self-assembled nano-columnar structure. Finally we have also demonstrated morphology induced bandgap tuning of one-dimensional nanostructures of Cu2O. Strong visi-



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ble emission by PL studies establishes that this kind of material will have great impacts on optoelectronic based devices such as fluorescent bio-labelling, LEDs etc.

edged for his valuable suggestions during the experimental studies.

AUTHOR INFORMATION

Supporting Information Available

Corresponding Author

A schematic of position of gun and substrate holder during normal and oblique angle sputter deposition, EDX spectra, SEM images of Cu2O films grown at Φ = 1 rpm at different angle of incidence, Reflectance spectra of samples deposited at 0º and 85º with varying substrate rotation, Relation of refractive index with wavelength for the samples grown at αD = 0º and 85º with varying substrate rotation, Transmittance spectra of Cu2O samples deposited at different incidence angles, and RTPL spectra of samples grown at Φ = 25 rpm for different deposition angles are provided as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org.

Dr. Sriparna Chatterjee, Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Acharya Vihar, Bhubaneswar 751013, India E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT S. Chatterjee and I. Mukherjee are grateful to DST (IFA12CH-65), India for the financial assistance and Prof. B. K. Mishra, Director, IMMT Bhubaneswar for providing the support to undertake this work. Prof. Pushan Ayyub is acknowl-

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Morphology dependent band gap tuning of Cu2O nano‐column array and their photoluminescence property.


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Band Gap Tuning and Room-Temperature Photoluminescence of a

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