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dry-air microwave plasma
Materials Science in Semiconductor Processing 74 (2018) 203–209
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Cu2O thin films obtained from sol-gel cuo films using a simple argon/dry-air microwave plasma
MARK
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M.A. Badillo-Ávilaa, R. Castanedo-Péreza, , G. Torres-Delgadoa, J. Márquez-Marína, O. Zelaya-Ángelb a b
Centro de Investigación y de Estudios Avanzados del I.P.N., Unidad Querétaroó, Querétaro 76230, Querétaro, Mexico Depto. de Física, Centro de Investigación y de Estudios Avanzados del I.P.N., A.P. 14-740, Ciudad de México 07360, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords: Cu2O by plasma Metastable copper Oxidation Reduction Home-made plasma
Cu2O thin films were obtained from CuO films using an argon/dry-air plasma treatment (PT) for times less than 30 s. CuO films were dip-coated on glass at a withdrawal speed of 8 cm/min from a sol-gel precursor solution. The as-prepared CuO samples were air-annealed conventionally at different temperatures, from 300 to 550 °C, to fabricate the CuO film-targets. An argon/dry-air PT was later applied to film-targets for 15, 20, 25 and 30 s in a modified microwave oven made with simple components. Cu2O mixed with a low amount of Cu was identified by X-ray diffraction in films treated for 20 s or less. Films only constituted of Cu2O were produced for most of the samples treated for 25 and 30 s. Depending on plasma processing time and CuO film-target annealing temperature, the crystallite size of Cu2O was largely changed (from 6 to 25 nm). Bigger Cu2O crystallite sizes were observed for targets with annealing of 400 °C. SEM studies showed surface morphology to depend on time of PT: grains grew, formed agglomerates and granules with longer exposure. These last caused a decrease in Cu2O film transparency below 800 nm for 30 s of treatment, but not for larger wavelengths nor for shorter times. Band gap value for CuO was 1.20 eV, and around 2.35 eV for Cu2O films below 25 s of plasma-treatment. However, 2.16 eV was calculated for the biggest crystallite size obtained at 30 s. Thickness and resistivity measurements were also performed. A strategic experiment showed that CuO films are reduced to metastable metallic copper with PT. This metallic copper readily oxidizes to Cu2O in open atmosphere. The biggest advantage of this plasma processing lies in the simplicity, short time of treatment and, low cost of the home-made equipment.
1. Introduction Copper oxide (I) is a desirable p-type semiconductor with good optical properties in the visible region. It has a band gap of 2.17 eV [1], which largely varies depending on the synthesis method and the crystallite size [2]. Cu2O has promising properties for solar cells, emitting diodes, photo-catalyst for hydrogen production, photo-catalyst for water and air decontamination, sensor for organic molecules, among others [3–7]. Cu2O can be synthesized by using chemical techniques such as sol-gel and precipitation, or from physical deposition methods as sputtering. Copper oxide (I) has also been prepared by controlled thermal oxidation of metallic copper. Less commonly, Cu2O is obtained from direct reduction of the more stable copper oxide (II), CuO [8]. The immediate advantage of the use of CuO as starting material lies in its synthesis simplicity and low cost of preparation. There are few reports in literature about the use of plasma processing to obtain Cu2O films. Most frequently, Cu2O is produced from
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metallic copper by applying an oxidant plasma, usually composed of pure oxygen, or oxygen mixed with argon [9–13]. Also, plasmas of pure nitrogen, or nitrogen mixed with hydrogen, have been used to reduce CuO and to convert it to Cu2O [5,14]. The copper source is regularly provided as metallic plates, it is also originated from sputtering deposition of metallic copper films. Though Cu2O can be obtained by direct use of an oxidant plasma on Cu, most usually samples are first treated thermally to obtain CuO. This copper oxide (II) is subsequently treated under a reducing plasma to obtain Cu2O. Although CuO could come from sol-gel synthesis, there are not reports for its direct use in plasma systems. Plasma treatments (PT) can be done in low pressure chambers (4–125 mTorr) or at atmospheric pressure. In some reports, PT's have been carried out using microwaves (300–800 W) [11,13,14], radio frequency (20–80 W) [10] and by inductively coupled plasmas (150–2000 W) [5,12]. Processing time varies widely, it could be as short as a minute to obtain Cu2O nanotubes [5] and as much as 2 h to
Corresponding author. E-mail address: [email protected] (R. Castanedo-Pérez).
http://dx.doi.org/10.1016/j.mssp.2017.10.036 Received 23 August 2017; Received in revised form 22 October 2017; Accepted 22 October 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Microwave plasma equipment scheme: a) treatment chamber and b) electrical circuitry.
fabricate Cu2O films [12]. Nevertheless, for both oxidant and reducing plasmas, mean time of treatment is 10 min. Moreover, Cu2O films of small thickness (80–130 nm) and thicker (400–600 nm) have been obtained from plasma processing. As it happens with other copper oxide transformation techniques, Cu2O frequently appears mixed with CuO and/or Cu; depending on the nature of the plasma used. In the case of oxidant plasmas, X-ray diffraction (XRD) measurements typically show small, but not negligible, signals of CuO. Also reducing plasmas sometimes produce residues of metallic copper. Nonetheless, for many reducing plasmas, Cu XRD peaks are observed, probably because copper plates are used as substrates and as film-targets for processing. For this reason, it is not clear if crystals of metallic copper are only allocated in the substrate or also within the produced Cu2O. In 2008, S. Han et. al. reported having produced Cu2O, from CuO, using an 800 W pure-nitrogen microwave plasma torch [14]. Their elaborated equipment, and its operation, is reported elsewhere [10]. To form a stable plasma, nitrogen flows of 11 and 1 l per minute were injected, perpendicular to each other, to an atmospheric-pressure chamber. The microwave-plasma treatment of CuO films produced Cu2O films in 10 min. CuO films were obtained from sputtered copper films on glass annealed at 500 °C for 12 h. In a 2015 conference proceedings book, the same authors reported the use of a copper acetate solution to produce CuO and Cu2O after plasma annealing of the spincoated organic precursors [15]. The same equipment, along with nitrogen and oxygen, were used. Nonetheless, Cu2O was obtained, in 20 min, only when not using oxygen in a second phase of plasma treatment. In this work, the conversion of CuO thin films to Cu2O ones, via an argon/dry-air microwave plasma, is studied. The process is carried out in a home-made equipment built with low-cost components, such as a commercial microwave oven (1500 W) and a low-grade vacuum pump. The fast plasma processing takes place inside a quartz tube in a low vacuum. Argon, nitrogen and oxygen are injected, in sum, with a mass flow of 12 milliliters per minute. The treatment is applied for no longer than 30 s. CuO film-targets are easily produced from a sol-gel solution and the dip-coating method on glass substrates. Depending on the annealing temperature of CuO films and time of PT, Cu2O, Cu or a mixture of both can be obtained. Moreover, the effects of time of plasma treatment on crystallite size, surface morphology and band gap, among others, were studied. Interestingly, films only constituted of Cu2O are produced from a metastable form of metallic copper only after the PT. This partial oxidation of metallic copper is driven by oxygen availability in open atmosphere. To our knowledge, the phenomenon has not been reported before. Based in these findings, this work is not only relevant in the simplification of processing procedures, but also in
copper oxide transformation phenomena. 2. Experimental details 2.1. CuO film-targets CuO thin films were deposited, on glass substrates by the dipcoating technique, from a homogeneous copper acetate solution. Copper acetate (Aldrich, 98%) was added to methanol (J. T. Baker, 99.8%) under fast stirring. Then, lactic acid (J. T. Baker, 88.6%) and glycerol (Merck, 85%) were incorporated to the mixture. To fully dissolve the copper salt, triethylamine (Merck, 98%) was used. All reagents were added in molar proportions of 1.0/74.25/1.42/0.34/1.0, respectively. Every film consisted of 5 coatings deposited at a withdrawal speed of 8 cm/min and dried in open atmosphere at a temperature of 260 °C for five minutes each. Immediately after deposition, CuO films were pre-annealed at 250 °C for one hour to burn organic compounds. Different groups of samples were later annealed at different temperatures (TA), from 300 °C to 550 °C in increments of 50 °C, for one hour in open atmosphere. 2.2. Plasma treatment system A plasma processing equipment, shown in Fig. 1a), was built by modifying a commercial 1500 W microwave oven (MWO). A circular cut on one of the oven's sides was made. The opening allows for a onefused-end-tube quartz (25 cm in length and 5.1 cm in diameter) to be allocated partially inside the oven chamber. Out of the oven, the open end of the quartz tube connects to a sealing metallic cap. The metallic cap has proper connections that allow target introduction, vacuum tubing and gas inlet. Changes to the electronics of the commercial MWO where made. A non-modified MWO has only one transformer that feeds a magnetron. Microwaves (MWs) are produced by the magnetron after the filament (cathode) emits electrons that are then accelerated. Nevertheless, the emission of MWs is not immediate because the filament takes some seconds to be heated up to produce electrons. To solve the problem, two independent transformers were accommodated as shown in Fig. 1b) [16]. The first transformer creates an electric potential difference of 3.3 V, which produces electrons by heating up the filament; a simple switch is used for this task. Once energized, the second transformer rapidly accelerates the electrons and produces a voltage difference of 2400 V, which is doubled in the magnetron. In this way, by first heating up the filament for some seconds, microwaves are generated instantaneously when full power is fed. An electrical circuit, based on a NOVUS N1100 controller, was designed and built to accurately control 204
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that the amount of metallic copper should increase with tP [17]. However, pure Cu2O is obtained above 20 s of treatment for almost all CuO targets, as full shaded symbols in Fig. 3 demonstrate. Remarkably, Cu2O films with traces of Cu (left shaded symbols) are produced in samples processed for 20 s or less. This phenomenon is explained in more detail in Section 4. It is worth noting, though, that this plasma also has a slightly oxidant nature because of the oxygen contained in dry-air. An interesting effect of the combination of TA and tP is also observed in Fig. 3. From 300 °C to 400 °C, there is a slight increase in crystal size for Cu2O for all plasma times, except 25 s. However, at 400 °C there is an inflexion point at which crystal size begins to rapidly decrease with TA. This behavior contrasts with the one presented by CuO film-targets. For plasma-produced Cu2O films, the annealing effect of CuO film-targets on crystal size had not been addressed before in literature. Because of the crystallite size maxima observed at 400 °C, this set of samples was further studied.
the exposure time of samples to plasma. CuO film-targets are introduced to the plasma chamber via the metallic cap. The CuO films are placed on a ceramic plate, with a cut-off window, that lets both substrate sides to receive the same plasma treatment. The chamber is sealed and a 11.25 Torr vacuum is reached by a diaphragm pump (KNF, model UN810FTP), the vacuum is kept for 5 min. Then, a mass flow controller (MKS, Type 247) allows for a continuous argon flow of 60 sccm and 60 sccm of dry-air; vacuum and gas flows are kept for other 5 min. To obtain Cu2O, the filament was heated for 10 s before CuO film-targets were plasma treated for times (tP) of 15, 20, 25 or 30 s at full power. Immediately after treatment, the vacuum pump and the argon flow were shut down, but the 60 sccm dryair flow was maintained for 15 min. Finally, samples were taken out of the quartz tube and stored in open atmosphere. 2.3. Characterization A Rigaku D/Max 2100 diffractometer (kα1 = 1.54 Å) was used to determine the crystalline phases present in each sample. The measurements, made at glancing angle, were used to calculate crystallite sizes (CS). For this, JADE 6.5® analysis software was used along with the corresponding XRD powder patterns for Cu (PDF #04-0836), Cu2O (PDF #05-0667) and CuO (PDF #48–1548). Scanning Electron Microscopy (SEM) images were collected by a JEOL electron probe microanalyzer (JXA-8530F). Transmittance (T) and reflectance (R) measurements were performed by an Agilent Cary-5000 spectrophotometer in the 190–2000 nm range, and in an Agilent 8453 spectrophotometer from 190 to 1100 nm (only transmission). Thicknesses were partly measured by a Dektak II profilometer, and partly by T-R data fitting in Film Wizard® optical analysis software. A Loresta GP MCP-T600 system was used to determine the resistivity (ρ) of selected samples.
3.2. Surface morphology and transmittance-reflectance measurements CuO target-films can be entirely transformed to Cu2O using the plasma treatment. In Fig. 4 (left), a characteristic brown color is observed for a CuO film annealed at 400 °C; once transformed (Fig. 4 right), an evidently yellow color is observed for the Cu2O film. The Cu2O film looks smooth and uniform, though the edges appear slightly darker; this can be a consequence of charge built up during the plasma treatment, which heats up the sample a little more at the edges. SEM micrographs of CuO targets with TA = 400 °C, and treated by plasma are exposed in Fig. 5. Without PT (Fig. 5a)), the CuO thin film surface appears uniform, though not completely smooth. The latter due to small particles that lye on the surface. When plasma is applied for 15 s, the surface appears more uniform with no particles (Fig. 5b) Moreover, grains seem to decrease slightly in size in comparison to a CuO film without treatment. For a film treated for 20 s (Fig. 5c)), the grain sizes are bigger and flat aggregates are formed on the surface. The aggregates do not have a well-defined shape, but they seem to have formed from the grains below; besides, they are distributed all over the surface, as shown in Fig. 5f). Evenly dispersed granules are observed attached to the surface in the sample treated for 25 s (Fig. 5d)). Though the surface looks blurry, the micrographs show the granules on focus; this might mean they are forming vertical columns [5]. For 30 s of treatment (Fig. 5e)), the film surface changes a lot; particles, fused agglomerates, and granules of different shapes and sizes are observed. From the image is obvious that a smooth surface is no longer existent. It can be suggested, from SEM images and XRD patterns, that small Cu2O grains are formed upon oxygen depletion from CuO films under plasma. Cu2O grains grow and fuse with a longer exposure to plasma, forming aggregates and granules. The granules seem to be forming stacks that give rise to columns. As tP reaches 30 s of treatment, the surface becomes irregular due to a larger fusion of aggregates and granules. In Fig. 6, the transmittance spectra of samples treated under plasma are shown; a 400 °C as-annealed CuO film-target spectrum has been added as comparison. The CuO film shows decreasing transmittance from the visible to the ultraviolet region. This is a characteristic that gives CuO films a dark brown color. In less than a day, samples treated in the argon/dry-air plasma become greenish-yellow or just yellow; which is associated with the presence of Cu2O as majority phase. For all CuO plasma-processed samples, an abrupt blue shift is observed, which is in accordance with the presence of Cu2O. The transmittance spectra for the films treated for 15 and 20 s are practically the same. For 25 and 30 s of treatment, below 600 nm, a decrease in transmittance is detected. However, the trend is reversed above 600 nm. Reflectance spectra are also showed in Fig. 6. Again, films treated for 15 and 20 s exhibit similar values. Then, the reflectance diminishes slightly for 25 s, but more importantly for 30 s of plasma. Samples treated for 15, 20 and
3. Results and discussion 3.1. Effect of time of plasma on crystalline structure and crystallite size of copper oxides The production of polycrystalline Cu2O thin films from CuO filmtargets, via a fast argon/dry-air PT, is demonstrated in XRD patterns of Fig. 2. CuO is the only polycrystalline phase in films with TA = 400 °C and tP = 0 s. The Cu2O pattern appears just after CuO film-targets are treated for 15 s. The intensity of diffraction peaks of Cu2O increases as tP rises to 20, 25 and 30 s. Although there is no signal from CuO phase, diffractograms do present a small amount of metallic copper, but mostly in the samples treated for than 20 s or less. Crystallite mean size (CS) was calculated from full width at half maximum (FWHM(S)) from corresponding XRD patterns and the Scherrer's formula (Eq. (1)). A silicon wafer diffractogram was used to compensate for instrumental error and to calibrate JADE 6.5® analysis software. A value of 0.94 was used as the k shape factor [8].
CS =
k×λ FWHM (S ) × cos θ
(1)
Results of crystal size calculations are shown in Fig. 3. For CuO filmtargets, crystallite sizes of 7.3 nm are found for TA = 300 °C, which slightly increase to 10.3 nm for 550 °C. When these targets are plasma treated for 15, 20, 25 or 30 s, they transform to Cu2O and usually attain a bigger crystal size. The latter not being true for TA′s above 400 °C and tP′s of 20 s or less. It is evident that crystallite sizes for Cu2O are bigger when tP of CuO targets is increased. Even though the TA also plays a role in determining the crystal size of Cu2O, its effect is less strong than tP. Moreover, the smaller Cu2O crystallite sizes obtained from CuO targets annealed at higher TA′s, might be due to the chemical stability achieved for CuO phase. It could be expected, from the reducing nature of the plasma used, 205
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50
60
70
80
20
Cu
30
40
50
60
Intensity (a. u)
270 0
a) 0 s
270
d) 25 s
540 270 0
c) 20 s 540 270 0
20
30
40
50
60
70
80
20
30
Cu2O
40
50
60
(311)
(220)
(111)
CuO (020)
(111)
(11-1)
0
(200)
Intensity (a. u)
b) 15 s
540
80
270 0
540
70
e) 30 s
540
(220)
40
(200)
30
(111)
20
70
80
2θ
2θ
Fig. 2. XRD patterns of CuO film-targets annealed at 400 °C and plasma-treated for a) 0, b) 15, c) 20, d) 25 and, e) 30 s.
24
CS (nm)
20
16
e) d)
c)
12
8
250
b) a) 300
350
400
450
500
550
TA (ºC) Fig. 3. Dependence on crystallite size for CuO and Cu2O for different TA′s and tP′s of: a) 0, b) 15, c) 20, d) 25, and, e) 30 s. Left shaded symbols indicate mixtures of Cu2O + Cu; full shaded symbols indicate pure Cu2O. Empty symbols indicate pure CuO (film-targets).
Fig. 4. CuO target-film to the left and plasma-transformed Cu2O film to the right. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
25 s show good transparency above 600 nm (R + T ≈ 100%). Nonetheless, Cu2O obtained at 30 s does not show good transparency until 800 nm; this phenomenon might be caused by light scattering associated to the size and shape of agglomerates (Fig. 5e)). The reflectance below 500 nm allows to compare the surface roughness among films [18]. In comparison to samples processed for shorter tP′s, a moderate reduction in reflectance is observed for the film treated for 25 s. The reflectance decrease is biggest for the sample treated for 30 s, which indicates a highly rough surface. These observations agree with the SEM micrographs already presented in Fig. 5.
3.3. Thickness, band gap and resistivity measurements Profilometry was useful to find the thickness of the CuO film-target (superscript “P” in Table 1), but only for one of the Cu2O films. The measurement requires an abrupt step that clearly differentiates glass substrate from thin film surface. Steps were easily made for CuO films by using HCl diluted in water at a 1:1 proportion. However, a clearly defined step could not be made for the Cu2O films treated above 15 s. In this case, the material could not be completely removed from the glass
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Fig. 5. SEM images of CuO film-targets annealed at 400 °C and plasma-treated for a) 0, b) 15, c) 20, d) 25, e) 30; and f) 20 s at lower magnification.
Reflectance, Transmittance (%)
80
Table 1 Estimated thicknesses and band gaps for CuO film-targets with TA = 400 °C and treated for different tP′s. Superscript “P” stands for profilometry, “FW” for Film Wizard® and “*” for proposed thickness. Resistivity measurements are also included.
TA = 400 ºC
T 60
a) 00 s b) 15 s c) 20 s d) 25 s e) 30 s
40
R
Time (s)
Phase
Thickness (nm)
Eg (eV)
ρ (102 Ω cm)
0 15 20 25 30
CuO Cu2O+Cu Cu2O+Cu Cu2O Cu2O
111.6P 103.7P, 102.1FW 102.4FW 102.85FW 102*
1.20 2.34 2.37 2.36 2.16
1.7 21.0 15.7 5.6 17.9
20
model was used. The transmittance-reflectance spectra of Fig. 6 were fitted using Film Wizard® optical software. The model (1 oscillator in SCI model + EMA-Bruggeman approximation) consisted of a single coating of pure Cu2O on glass substrate and roughness was taken as part of the thickness. Data below 450 nm was not considered for fitting. Using the model, the thicknesses for all, but one, of the Cu2O films were estimated (superscript “FW” in Table 1). The estimates were coherent among themselves and in agreement for the Cu2O sample treated for 15 s. It was not possible to fit the data for the film treated at 30 s, this because of a high scattering of light that would require a much more complex optical model. The calculations show that thickness decreases by around 9 nm for CuO films upon 15 s of PT. The decline might be due to the loss of
0 400
800
1200
1600
2000
Wavelength (nm) Fig. 6. Transmittance-reflectance spectra for CuO film-targets annealed at 400 °C and plasma treated for a) 0, b) 15, c) 20, d) 25 and, e) 30 s. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
substrate by the same procedure, which was observed as gray traces on the glass surface. To estimate the thicknesses of the other Cu2O films, a simple optical 207
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oxygen. The thicknesses seem to be the same for samples treated for 15, 20 and 25 s. For the sake of comparison, a thickness of 102 nm was used for calculations involving the sample treated for 30 s. Transmission spectra and thicknesses (d) were used for band gap calculation. The absorption coefficient, α, was estimated by using the expression α = ln(T)/d. Energy gaps were determined from the Taucplot, in which (α E)n is plotted against E, by intersecting the E axis with a linear fit around the fundamental absorption region. Depending on the nature of the material, different fundamental transition values are assigned to n. For indirect band gap materials, as it is the case for CuO, n has a value of 1/2. For Cu2O, the fundamental transition is direct forbidden, which has an n value of 2/3 [8]. It can be seen, from Table 1, that CuO annealed at 400 °C has a band gap of 1.20 eV, which is consistent with reported values of sputtered CuO films [19]. When CuO film-targets are plasma treated for only 15 s, a band gap of 2.34 eV is obtained. The sudden increase in band gap energy clearly shows the conversion of CuO to Cu2O by means of the plasma. As Martinez et al. explain in reference [8], band gap reported values for Cu2O largely vary, from 2.0 for sol-gel synthesis to 2.5 eV for sputtering deposition. The differences arise from contribution of several mechanisms and from Cu2O film thicknesses. Band gap values of 2.37 and 2.36 eV are calculated for tP′s of 20 and 25 s, respectively. For tP = 30 s a band gap of 2.16 eV is attained; this value is close to the one ascribed for a natural Cu2O single crystal [1]. The difference in energy gaps, between the films treated for 30 s and the samples with less time of processing, could be attributed to the effect of crystallite size. In reference [2], Chang et. al reported a threshold around 14 nm of crystal size below which band gap was about 2.35 eV; above the threshold, band gap was estimated to be 2.17 eV. This effect was also reported, by Wei-Hong Ke et. al., for very small Cu2O nanocubes, which sizes are controlled by chemical means [20]. A similar trend was also observed for the present work, though apparently at 19 nm as threshold (Fig. 3). Resistivity measurements using the four-probe method were also performed. From Table 1, it is seen that CuO film-targets annealed at 400 °C have a ρ = 1.7×102 Ω cm, a higher value (104 Ω cm) was reported in previous work for the same synthesis. However, for such study the thermal treatment of CuO was at 250 °C, which may explain the difference. Upon PT for 15 s, the resistivity increases rapidly to 21.2 × 102 Ω cm for Cu2O + Cu. With further tP, ρ decreases to 15.7 × 102 Ω cm for 20 s. This initial decline in resistivity, with growing exposure to plasma, could be owed to the growth of crystal size (Fig. 3). It is interesting that the resistivity of Cu2O + Cu films is bigger than that of the CuO target, this despite Cu2O has traces of metallic copper for 15 and 20 s of treatment. In this last regard, metallic copper might be dispersed in the film given its low presence, which does not add to conductivity. For the film treated for 25 s, in which there is not metallic copper, the lowest ρ value of 5.6 × 102 Ω cm is obtained. This might be attributed to Cu2O alone. On the other hand, though Cu2O films obtained at 30 s of treatment have the biggest crystallite size, they conversely show an increase in resistivity. This effect might be due to the highly disordered array of agglomerates, which would be causing the rise in such parameter.
Fig. 7. XRD patterns of a CuO film-target a) just after plasma treatment for 20 s, and b) 1 day exposed to open atmosphere.
of removing or adding oxygen to the treated material. In this regard, the full transformation of CuO, or Cu, to Cu2O is achieved during the PT. Nonetheless, the transformation of CuO to Cu2O can occur by a different mechanism, as evidenced in this report. All the samples presented in this work were measured at least a day after the plasma processing. The transformation of films to Cu2O starts in the PT; however, full conversion is achieved afterwards. CuO filmtargets are dark brown in color, but they become dark-gray, dark green or gray-yellow just after the PT. Films are stored in open atmosphere and after some hours or a day, it is evident they suffer a visible change of color to yellow or greenish yellow. To better illustrate the described phenomena, a CuO film-target with TA = 450 °C was treated with plasma for 25 s. For this specific case, all gases were shut down after processing; the sample was still cooled down for 15 min. XRD and transmittance-reflectance measurements were performed immediately after the PT, as well as a day later. It can be seen, from Fig. 7a), that Cu2O with a crystallite size = 17.5 ± 1.4 nm, and Cu are obtained just after the PT of the CuO filmtarget. The treated sample is left exposed to open atmosphere. According to Fig. 7b), the signal from metallic copper almost disappears after a day. Nevertheless, the counts for Cu2O increase while the crystal size remains virtually the same, 17.1 ± 0.9 nm. The XRD patterns suggest that an important proportion of metastable metallic copper is formed by the PT. This Cu easily oxidizes, at ambient temperature in the presence of oxygen, to produce crystalline Cu2O. The described changes are also observable in the transmittance-reflectance spectra of Fig. 8. In comparison to the same sample measured just after the PT, the film becomes much more transparent and less reflecting after a day. The higher transmittance can be assigned to Cu2O as the majority phase, while the lower transmittance is mainly due to reflecting metallic copper. Moreover, the sum of reflectance + transmittance is less than 40% for the film just plasma-treated. The last indicates that a high proportion of metallic copper is absorbing light, that would explain the gray or dark color observed for samples just after the plasma processing. After a day, when copper oxidizes, reflectance + transmittance increases to 75%, which means there is a lower proportion of Cu. The evidence presented suggests that Cu2O is being obtained after
4. CuO reduction phenomena by plasma treatment The conversion mechanism of CuO to Cu2O has been discussed before in literature. Plasma spectroscopy has suggested the formation of several ionized and dissociated species, which are dependent on the gases used. In the reduction of CuO to Cu2O, nitrogen and hydrogen ionized species remove oxygen from CuO by forming nitrogen oxides and water [5,10,14]. When metallic copper plates are used as a source, an applied oxygen plasma produces Cu2O and/or CuO by absorbing oxygen. Ionized and dissociated oxygen species are also formed during these treatments. Moreover, argon is commonly used along with oxygen; its role as ionized species is not well understood, however [12]. The referenced reports explain the obtaining of Cu2O as a consequence 208
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01/256399 and CeMIE-Sol-PY207450/25. The authors also thanks CONACyT for the fellowship with No. 378107 awarded to Ms. C. Miguel Ángel Badillo Ávila. Authors thank the technical assistance of Bs. C. Dinora Coria Quiñones and Saúl Velázquez Rivera. Special thanks are due to Adair Jiménez Nieto for SEM morphology measurements.
45
30
References [1] H. Matsumoto, K. Saito, M. Hasuo, S. Kono, N. Nagasawa, Revived interest on yellow-exciton series in Cu2O: an experimental aspect, Solid State Commun. 97 (1996) 125–129. [2] Y. Chang, J.J. Teo, H.C. Zeng, Formation of colloidal CuO nanocrystallites and their spherical aggregation and reductive transformation to hollow Cu2O nanospheres, Langmuir 21 (2005) 1074–1079. [3] A.S. Zoolfakar, R.A. Rani, A.J. Morfa, A.P. O’Mullane, K. Kalantar-zadeh, Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications, J. Mater. Chem. C 2 (2014) 5247–5270. [4] Y. Bessekhouad, D. Robert, J. Weber, Bi2S3/TiO2 and CdS/TiO2 heterojunctions as an available configuration for photocatalytic degradation of organic pollutant, J. Photochem. Photobiol. A Chem. 163 (2004) 569–580. [5] R.C. Wang, H.Y. Lin, Efficient surface enhanced Raman scattering from Cu2O porous nanowires transformed from CuO nanowires by plasma treatments, Mater. Chem. Phys. 136 (2012) 661–665. [6] C.S. Tan, S.C. Hsu, W.H. Ke, L.J. Chen, M.H. Huang, Facet-dependent electrical conductivity properties of Cu2O crystals, Nano Lett. 15 (2015) 2155–2160. [7] C.-Y. Chu, M.H. Huang, Facet-dependent photocatalytic properties of Cu2O crystals probed by using electron, hole and radical scavengers, J. Mater. Chem. A5 (2017) 15116–15123. [8] G. Martinez-Saucedo, R. Castanedo-Perez, G. Torres-Delgado, A. Mendoza-Galvan, O.Z. Angel, Cuprous oxide thin films obtained by dip-coating method using rapid thermal annealing treatments, Mater. Sci. Semicond. Process. 68 (2017) 133–139. [9] C. Ooi, G.K.L. Goh, Formation of cuprous oxide films via oxygen plasma, Thin Solid Films 518 (2010) e98–e100. [10] M.J. Chen, C.-Y. Wu, Y.M. Kuo, H.Y. Chen, C.H. Tsai, Preparation of Cu2O nanowires by thermal oxidation-plasma reduction method, Appl. Phys. A 108 (2012) 133–141. [11] K.V. Rajani, S. Daniels, E. Mcglynn, R.P. Gandhiraman, R. Groarke, P.J. Mcnally, Low temperature growth technique for nanocrystalline cuprous oxide thin films using microwave plasma oxidation of copper, Mater. Lett. 71 (2012) 160–163. [12] W. Wu, et al., Characterization of Cu2O and Cu2O/Ag2O thin films synthesized by plasma oxidation, Vaccum 118 (2015) 147–151. [13] Y.M. Chan, Y.T. Wu, S. Jou, Oxide solar cells fabricated using zinc oxide and plasma-oxidized cuprous oxide, Jpn. J. Appl. Phys. 51 (2012) 125502-1–125502-4. [14] S. Han, H.Y. Chen, L.T. Kuo, C.H. Tsai, Characterization of cuprous oxide films prepared by post-annealing of cupric oxide using an atmospheric nitrogen pressure plasma torch, Thin Solid Films 517 (2008) 1195–1199. [15] H. Chen, J. Mai, Preparation and characterization of cuprous oxide thin films by an atmospheric pressure plasma annealing, in: Proceedings of the 22nd International Symposium on Plasma Chemistry, 2015, pp. 9–10. [16] J.M. Criado, M.J. Diánez, L.A.Pérez. Maqueda, Modificación de un horno doméstico de microondas para el tratamiento de materiales a alta temperatura bajo atmósfera controlada, in Congreso Conamet/SAM, 2008. [17] S. Tajima, S. Tsuchiya, M. Matsumori, S. Nakatsuka, T. Ichiki, High-rate reduction of copper oxide using atmospheric-pressure inductively coupled plasma microjets, Thin Solid Films 519 (2011) 6773–6777. [18] H.E. Bennett, J.O. Porteus, Relation between surface roughness and specular reflectance at normal incidence, J. Opt. Soc. Am. 51 (1961) 123–129. [19] S. Rehman, A. Mumtaz, S.K. Hasanain, Size effects on the magnetic and optical properties of CuO nanoparticles, J. Nanopart. Res. 13 (2011) 2497–2507. [20] W.H. Ke, C.F. Hsia, Y.J. Chen, M.H. Huang, Synthesis of ultrasmall Cu2O nanocubes and octahedra with tunable sizes for facet-dependent optical property examination, Small (2016) 3530–3534. [21] A.E. Rakhshani, Preparation, characteristics and photovoltaic properties of cuprous oxide – a review, Solid. State Electron. 29 (1986) 7–17.
15
R, Just after P. R, 1 day after T, Just after P. T, 1 day after 0 200
400
600
800
1000
Wavelength (nm) Fig. 8. Optical spectra of a CuO film-target a) just after tP of 20 s, and b) 1 day exposed to open atmosphere.
the metastable metallic copper oxidizes. It is intriguing, though, that Cu2O instead of CuO is achieved by the copper oxidation. The last given that CuO has been reported to be the most stable oxide of copper at ambient conditions [21]. This unusual oxidation process provides an opportunity to further study the effects of after-plasma-treatment parameters. Current research is under development. 5. Conclusions In less than 30 s, Cu2O thin films were produced from an argon/dryair plasma treatment of CuO films. CuO films were prepared from solgel synthesis by dip coating, which had not been previously reported to be used in plasma systems. Conventional annealing temperatures of CuO film-targets showed that plasma-produced Cu2O crystallite sizes are bigger when lower TA′s are used. However, there was an optimal TA around 400 °C for which crystallite size was maximized to 25 nm for 30 s of treatment. Energy gap values were determined to be around 2.35 eV for Cu2O samples produced at less than 25 s. Nonetheless, for 30 s of treatment a band gap of 2.16 was found, which was practically the same that the one reported for a single Cu2O crystal. The lowest resistivity of a Cu2O film, obtained at 25 s of plasma processing, was found to be 5.6 × 102 Ω cm. Most importantly, it was proved that an argon/dry-air PT reduces CuO to a metastable form of crystalline metallic copper; in less than a day in open atmosphere, metallic copper gradually oxidizes and transforms to crystalline Cu2O. This oxidation phenomenon had not been reported before, but it could be used to tailor many of the properties of Cu2O. Acknowledgments This work was supported by Consejo Nacional de Ciencia y Tecnología (CONACyT) under projects CB-2013-01/222909, CB-2015-
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Cu2O thin films obtained from sol-gel cuo films using a simple argon/dry-air microwave plasma
MARK
⁎
M.A. Badillo-Ávilaa, R. Castanedo-Péreza, , G. Torres-Delgadoa, J. Márquez-Marína, O. Zelaya-Ángelb a b
Centro de Investigación y de Estudios Avanzados del I.P.N., Unidad Querétaroó, Querétaro 76230, Querétaro, Mexico Depto. de Física, Centro de Investigación y de Estudios Avanzados del I.P.N., A.P. 14-740, Ciudad de México 07360, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords: Cu2O by plasma Metastable copper Oxidation Reduction Home-made plasma
Cu2O thin films were obtained from CuO films using an argon/dry-air plasma treatment (PT) for times less than 30 s. CuO films were dip-coated on glass at a withdrawal speed of 8 cm/min from a sol-gel precursor solution. The as-prepared CuO samples were air-annealed conventionally at different temperatures, from 300 to 550 °C, to fabricate the CuO film-targets. An argon/dry-air PT was later applied to film-targets for 15, 20, 25 and 30 s in a modified microwave oven made with simple components. Cu2O mixed with a low amount of Cu was identified by X-ray diffraction in films treated for 20 s or less. Films only constituted of Cu2O were produced for most of the samples treated for 25 and 30 s. Depending on plasma processing time and CuO film-target annealing temperature, the crystallite size of Cu2O was largely changed (from 6 to 25 nm). Bigger Cu2O crystallite sizes were observed for targets with annealing of 400 °C. SEM studies showed surface morphology to depend on time of PT: grains grew, formed agglomerates and granules with longer exposure. These last caused a decrease in Cu2O film transparency below 800 nm for 30 s of treatment, but not for larger wavelengths nor for shorter times. Band gap value for CuO was 1.20 eV, and around 2.35 eV for Cu2O films below 25 s of plasma-treatment. However, 2.16 eV was calculated for the biggest crystallite size obtained at 30 s. Thickness and resistivity measurements were also performed. A strategic experiment showed that CuO films are reduced to metastable metallic copper with PT. This metallic copper readily oxidizes to Cu2O in open atmosphere. The biggest advantage of this plasma processing lies in the simplicity, short time of treatment and, low cost of the home-made equipment.
1. Introduction Copper oxide (I) is a desirable p-type semiconductor with good optical properties in the visible region. It has a band gap of 2.17 eV [1], which largely varies depending on the synthesis method and the crystallite size [2]. Cu2O has promising properties for solar cells, emitting diodes, photo-catalyst for hydrogen production, photo-catalyst for water and air decontamination, sensor for organic molecules, among others [3–7]. Cu2O can be synthesized by using chemical techniques such as sol-gel and precipitation, or from physical deposition methods as sputtering. Copper oxide (I) has also been prepared by controlled thermal oxidation of metallic copper. Less commonly, Cu2O is obtained from direct reduction of the more stable copper oxide (II), CuO [8]. The immediate advantage of the use of CuO as starting material lies in its synthesis simplicity and low cost of preparation. There are few reports in literature about the use of plasma processing to obtain Cu2O films. Most frequently, Cu2O is produced from
⁎
metallic copper by applying an oxidant plasma, usually composed of pure oxygen, or oxygen mixed with argon [9–13]. Also, plasmas of pure nitrogen, or nitrogen mixed with hydrogen, have been used to reduce CuO and to convert it to Cu2O [5,14]. The copper source is regularly provided as metallic plates, it is also originated from sputtering deposition of metallic copper films. Though Cu2O can be obtained by direct use of an oxidant plasma on Cu, most usually samples are first treated thermally to obtain CuO. This copper oxide (II) is subsequently treated under a reducing plasma to obtain Cu2O. Although CuO could come from sol-gel synthesis, there are not reports for its direct use in plasma systems. Plasma treatments (PT) can be done in low pressure chambers (4–125 mTorr) or at atmospheric pressure. In some reports, PT's have been carried out using microwaves (300–800 W) [11,13,14], radio frequency (20–80 W) [10] and by inductively coupled plasmas (150–2000 W) [5,12]. Processing time varies widely, it could be as short as a minute to obtain Cu2O nanotubes [5] and as much as 2 h to
Corresponding author. E-mail address: [email protected] (R. Castanedo-Pérez).
http://dx.doi.org/10.1016/j.mssp.2017.10.036 Received 23 August 2017; Received in revised form 22 October 2017; Accepted 22 October 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Microwave plasma equipment scheme: a) treatment chamber and b) electrical circuitry.
fabricate Cu2O films [12]. Nevertheless, for both oxidant and reducing plasmas, mean time of treatment is 10 min. Moreover, Cu2O films of small thickness (80–130 nm) and thicker (400–600 nm) have been obtained from plasma processing. As it happens with other copper oxide transformation techniques, Cu2O frequently appears mixed with CuO and/or Cu; depending on the nature of the plasma used. In the case of oxidant plasmas, X-ray diffraction (XRD) measurements typically show small, but not negligible, signals of CuO. Also reducing plasmas sometimes produce residues of metallic copper. Nonetheless, for many reducing plasmas, Cu XRD peaks are observed, probably because copper plates are used as substrates and as film-targets for processing. For this reason, it is not clear if crystals of metallic copper are only allocated in the substrate or also within the produced Cu2O. In 2008, S. Han et. al. reported having produced Cu2O, from CuO, using an 800 W pure-nitrogen microwave plasma torch [14]. Their elaborated equipment, and its operation, is reported elsewhere [10]. To form a stable plasma, nitrogen flows of 11 and 1 l per minute were injected, perpendicular to each other, to an atmospheric-pressure chamber. The microwave-plasma treatment of CuO films produced Cu2O films in 10 min. CuO films were obtained from sputtered copper films on glass annealed at 500 °C for 12 h. In a 2015 conference proceedings book, the same authors reported the use of a copper acetate solution to produce CuO and Cu2O after plasma annealing of the spincoated organic precursors [15]. The same equipment, along with nitrogen and oxygen, were used. Nonetheless, Cu2O was obtained, in 20 min, only when not using oxygen in a second phase of plasma treatment. In this work, the conversion of CuO thin films to Cu2O ones, via an argon/dry-air microwave plasma, is studied. The process is carried out in a home-made equipment built with low-cost components, such as a commercial microwave oven (1500 W) and a low-grade vacuum pump. The fast plasma processing takes place inside a quartz tube in a low vacuum. Argon, nitrogen and oxygen are injected, in sum, with a mass flow of 12 milliliters per minute. The treatment is applied for no longer than 30 s. CuO film-targets are easily produced from a sol-gel solution and the dip-coating method on glass substrates. Depending on the annealing temperature of CuO films and time of PT, Cu2O, Cu or a mixture of both can be obtained. Moreover, the effects of time of plasma treatment on crystallite size, surface morphology and band gap, among others, were studied. Interestingly, films only constituted of Cu2O are produced from a metastable form of metallic copper only after the PT. This partial oxidation of metallic copper is driven by oxygen availability in open atmosphere. To our knowledge, the phenomenon has not been reported before. Based in these findings, this work is not only relevant in the simplification of processing procedures, but also in
copper oxide transformation phenomena. 2. Experimental details 2.1. CuO film-targets CuO thin films were deposited, on glass substrates by the dipcoating technique, from a homogeneous copper acetate solution. Copper acetate (Aldrich, 98%) was added to methanol (J. T. Baker, 99.8%) under fast stirring. Then, lactic acid (J. T. Baker, 88.6%) and glycerol (Merck, 85%) were incorporated to the mixture. To fully dissolve the copper salt, triethylamine (Merck, 98%) was used. All reagents were added in molar proportions of 1.0/74.25/1.42/0.34/1.0, respectively. Every film consisted of 5 coatings deposited at a withdrawal speed of 8 cm/min and dried in open atmosphere at a temperature of 260 °C for five minutes each. Immediately after deposition, CuO films were pre-annealed at 250 °C for one hour to burn organic compounds. Different groups of samples were later annealed at different temperatures (TA), from 300 °C to 550 °C in increments of 50 °C, for one hour in open atmosphere. 2.2. Plasma treatment system A plasma processing equipment, shown in Fig. 1a), was built by modifying a commercial 1500 W microwave oven (MWO). A circular cut on one of the oven's sides was made. The opening allows for a onefused-end-tube quartz (25 cm in length and 5.1 cm in diameter) to be allocated partially inside the oven chamber. Out of the oven, the open end of the quartz tube connects to a sealing metallic cap. The metallic cap has proper connections that allow target introduction, vacuum tubing and gas inlet. Changes to the electronics of the commercial MWO where made. A non-modified MWO has only one transformer that feeds a magnetron. Microwaves (MWs) are produced by the magnetron after the filament (cathode) emits electrons that are then accelerated. Nevertheless, the emission of MWs is not immediate because the filament takes some seconds to be heated up to produce electrons. To solve the problem, two independent transformers were accommodated as shown in Fig. 1b) [16]. The first transformer creates an electric potential difference of 3.3 V, which produces electrons by heating up the filament; a simple switch is used for this task. Once energized, the second transformer rapidly accelerates the electrons and produces a voltage difference of 2400 V, which is doubled in the magnetron. In this way, by first heating up the filament for some seconds, microwaves are generated instantaneously when full power is fed. An electrical circuit, based on a NOVUS N1100 controller, was designed and built to accurately control 204
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that the amount of metallic copper should increase with tP [17]. However, pure Cu2O is obtained above 20 s of treatment for almost all CuO targets, as full shaded symbols in Fig. 3 demonstrate. Remarkably, Cu2O films with traces of Cu (left shaded symbols) are produced in samples processed for 20 s or less. This phenomenon is explained in more detail in Section 4. It is worth noting, though, that this plasma also has a slightly oxidant nature because of the oxygen contained in dry-air. An interesting effect of the combination of TA and tP is also observed in Fig. 3. From 300 °C to 400 °C, there is a slight increase in crystal size for Cu2O for all plasma times, except 25 s. However, at 400 °C there is an inflexion point at which crystal size begins to rapidly decrease with TA. This behavior contrasts with the one presented by CuO film-targets. For plasma-produced Cu2O films, the annealing effect of CuO film-targets on crystal size had not been addressed before in literature. Because of the crystallite size maxima observed at 400 °C, this set of samples was further studied.
the exposure time of samples to plasma. CuO film-targets are introduced to the plasma chamber via the metallic cap. The CuO films are placed on a ceramic plate, with a cut-off window, that lets both substrate sides to receive the same plasma treatment. The chamber is sealed and a 11.25 Torr vacuum is reached by a diaphragm pump (KNF, model UN810FTP), the vacuum is kept for 5 min. Then, a mass flow controller (MKS, Type 247) allows for a continuous argon flow of 60 sccm and 60 sccm of dry-air; vacuum and gas flows are kept for other 5 min. To obtain Cu2O, the filament was heated for 10 s before CuO film-targets were plasma treated for times (tP) of 15, 20, 25 or 30 s at full power. Immediately after treatment, the vacuum pump and the argon flow were shut down, but the 60 sccm dryair flow was maintained for 15 min. Finally, samples were taken out of the quartz tube and stored in open atmosphere. 2.3. Characterization A Rigaku D/Max 2100 diffractometer (kα1 = 1.54 Å) was used to determine the crystalline phases present in each sample. The measurements, made at glancing angle, were used to calculate crystallite sizes (CS). For this, JADE 6.5® analysis software was used along with the corresponding XRD powder patterns for Cu (PDF #04-0836), Cu2O (PDF #05-0667) and CuO (PDF #48–1548). Scanning Electron Microscopy (SEM) images were collected by a JEOL electron probe microanalyzer (JXA-8530F). Transmittance (T) and reflectance (R) measurements were performed by an Agilent Cary-5000 spectrophotometer in the 190–2000 nm range, and in an Agilent 8453 spectrophotometer from 190 to 1100 nm (only transmission). Thicknesses were partly measured by a Dektak II profilometer, and partly by T-R data fitting in Film Wizard® optical analysis software. A Loresta GP MCP-T600 system was used to determine the resistivity (ρ) of selected samples.
3.2. Surface morphology and transmittance-reflectance measurements CuO target-films can be entirely transformed to Cu2O using the plasma treatment. In Fig. 4 (left), a characteristic brown color is observed for a CuO film annealed at 400 °C; once transformed (Fig. 4 right), an evidently yellow color is observed for the Cu2O film. The Cu2O film looks smooth and uniform, though the edges appear slightly darker; this can be a consequence of charge built up during the plasma treatment, which heats up the sample a little more at the edges. SEM micrographs of CuO targets with TA = 400 °C, and treated by plasma are exposed in Fig. 5. Without PT (Fig. 5a)), the CuO thin film surface appears uniform, though not completely smooth. The latter due to small particles that lye on the surface. When plasma is applied for 15 s, the surface appears more uniform with no particles (Fig. 5b) Moreover, grains seem to decrease slightly in size in comparison to a CuO film without treatment. For a film treated for 20 s (Fig. 5c)), the grain sizes are bigger and flat aggregates are formed on the surface. The aggregates do not have a well-defined shape, but they seem to have formed from the grains below; besides, they are distributed all over the surface, as shown in Fig. 5f). Evenly dispersed granules are observed attached to the surface in the sample treated for 25 s (Fig. 5d)). Though the surface looks blurry, the micrographs show the granules on focus; this might mean they are forming vertical columns [5]. For 30 s of treatment (Fig. 5e)), the film surface changes a lot; particles, fused agglomerates, and granules of different shapes and sizes are observed. From the image is obvious that a smooth surface is no longer existent. It can be suggested, from SEM images and XRD patterns, that small Cu2O grains are formed upon oxygen depletion from CuO films under plasma. Cu2O grains grow and fuse with a longer exposure to plasma, forming aggregates and granules. The granules seem to be forming stacks that give rise to columns. As tP reaches 30 s of treatment, the surface becomes irregular due to a larger fusion of aggregates and granules. In Fig. 6, the transmittance spectra of samples treated under plasma are shown; a 400 °C as-annealed CuO film-target spectrum has been added as comparison. The CuO film shows decreasing transmittance from the visible to the ultraviolet region. This is a characteristic that gives CuO films a dark brown color. In less than a day, samples treated in the argon/dry-air plasma become greenish-yellow or just yellow; which is associated with the presence of Cu2O as majority phase. For all CuO plasma-processed samples, an abrupt blue shift is observed, which is in accordance with the presence of Cu2O. The transmittance spectra for the films treated for 15 and 20 s are practically the same. For 25 and 30 s of treatment, below 600 nm, a decrease in transmittance is detected. However, the trend is reversed above 600 nm. Reflectance spectra are also showed in Fig. 6. Again, films treated for 15 and 20 s exhibit similar values. Then, the reflectance diminishes slightly for 25 s, but more importantly for 30 s of plasma. Samples treated for 15, 20 and
3. Results and discussion 3.1. Effect of time of plasma on crystalline structure and crystallite size of copper oxides The production of polycrystalline Cu2O thin films from CuO filmtargets, via a fast argon/dry-air PT, is demonstrated in XRD patterns of Fig. 2. CuO is the only polycrystalline phase in films with TA = 400 °C and tP = 0 s. The Cu2O pattern appears just after CuO film-targets are treated for 15 s. The intensity of diffraction peaks of Cu2O increases as tP rises to 20, 25 and 30 s. Although there is no signal from CuO phase, diffractograms do present a small amount of metallic copper, but mostly in the samples treated for than 20 s or less. Crystallite mean size (CS) was calculated from full width at half maximum (FWHM(S)) from corresponding XRD patterns and the Scherrer's formula (Eq. (1)). A silicon wafer diffractogram was used to compensate for instrumental error and to calibrate JADE 6.5® analysis software. A value of 0.94 was used as the k shape factor [8].
CS =
k×λ FWHM (S ) × cos θ
(1)
Results of crystal size calculations are shown in Fig. 3. For CuO filmtargets, crystallite sizes of 7.3 nm are found for TA = 300 °C, which slightly increase to 10.3 nm for 550 °C. When these targets are plasma treated for 15, 20, 25 or 30 s, they transform to Cu2O and usually attain a bigger crystal size. The latter not being true for TA′s above 400 °C and tP′s of 20 s or less. It is evident that crystallite sizes for Cu2O are bigger when tP of CuO targets is increased. Even though the TA also plays a role in determining the crystal size of Cu2O, its effect is less strong than tP. Moreover, the smaller Cu2O crystallite sizes obtained from CuO targets annealed at higher TA′s, might be due to the chemical stability achieved for CuO phase. It could be expected, from the reducing nature of the plasma used, 205
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50
60
70
80
20
Cu
30
40
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60
Intensity (a. u)
270 0
a) 0 s
270
d) 25 s
540 270 0
c) 20 s 540 270 0
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30
40
50
60
70
80
20
30
Cu2O
40
50
60
(311)
(220)
(111)
CuO (020)
(111)
(11-1)
0
(200)
Intensity (a. u)
b) 15 s
540
80
270 0
540
70
e) 30 s
540
(220)
40
(200)
30
(111)
20
70
80
2θ
2θ
Fig. 2. XRD patterns of CuO film-targets annealed at 400 °C and plasma-treated for a) 0, b) 15, c) 20, d) 25 and, e) 30 s.
24
CS (nm)
20
16
e) d)
c)
12
8
250
b) a) 300
350
400
450
500
550
TA (ºC) Fig. 3. Dependence on crystallite size for CuO and Cu2O for different TA′s and tP′s of: a) 0, b) 15, c) 20, d) 25, and, e) 30 s. Left shaded symbols indicate mixtures of Cu2O + Cu; full shaded symbols indicate pure Cu2O. Empty symbols indicate pure CuO (film-targets).
Fig. 4. CuO target-film to the left and plasma-transformed Cu2O film to the right. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
25 s show good transparency above 600 nm (R + T ≈ 100%). Nonetheless, Cu2O obtained at 30 s does not show good transparency until 800 nm; this phenomenon might be caused by light scattering associated to the size and shape of agglomerates (Fig. 5e)). The reflectance below 500 nm allows to compare the surface roughness among films [18]. In comparison to samples processed for shorter tP′s, a moderate reduction in reflectance is observed for the film treated for 25 s. The reflectance decrease is biggest for the sample treated for 30 s, which indicates a highly rough surface. These observations agree with the SEM micrographs already presented in Fig. 5.
3.3. Thickness, band gap and resistivity measurements Profilometry was useful to find the thickness of the CuO film-target (superscript “P” in Table 1), but only for one of the Cu2O films. The measurement requires an abrupt step that clearly differentiates glass substrate from thin film surface. Steps were easily made for CuO films by using HCl diluted in water at a 1:1 proportion. However, a clearly defined step could not be made for the Cu2O films treated above 15 s. In this case, the material could not be completely removed from the glass
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Fig. 5. SEM images of CuO film-targets annealed at 400 °C and plasma-treated for a) 0, b) 15, c) 20, d) 25, e) 30; and f) 20 s at lower magnification.
Reflectance, Transmittance (%)
80
Table 1 Estimated thicknesses and band gaps for CuO film-targets with TA = 400 °C and treated for different tP′s. Superscript “P” stands for profilometry, “FW” for Film Wizard® and “*” for proposed thickness. Resistivity measurements are also included.
TA = 400 ºC
T 60
a) 00 s b) 15 s c) 20 s d) 25 s e) 30 s
40
R
Time (s)
Phase
Thickness (nm)
Eg (eV)
ρ (102 Ω cm)
0 15 20 25 30
CuO Cu2O+Cu Cu2O+Cu Cu2O Cu2O
111.6P 103.7P, 102.1FW 102.4FW 102.85FW 102*
1.20 2.34 2.37 2.36 2.16
1.7 21.0 15.7 5.6 17.9
20
model was used. The transmittance-reflectance spectra of Fig. 6 were fitted using Film Wizard® optical software. The model (1 oscillator in SCI model + EMA-Bruggeman approximation) consisted of a single coating of pure Cu2O on glass substrate and roughness was taken as part of the thickness. Data below 450 nm was not considered for fitting. Using the model, the thicknesses for all, but one, of the Cu2O films were estimated (superscript “FW” in Table 1). The estimates were coherent among themselves and in agreement for the Cu2O sample treated for 15 s. It was not possible to fit the data for the film treated at 30 s, this because of a high scattering of light that would require a much more complex optical model. The calculations show that thickness decreases by around 9 nm for CuO films upon 15 s of PT. The decline might be due to the loss of
0 400
800
1200
1600
2000
Wavelength (nm) Fig. 6. Transmittance-reflectance spectra for CuO film-targets annealed at 400 °C and plasma treated for a) 0, b) 15, c) 20, d) 25 and, e) 30 s. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
substrate by the same procedure, which was observed as gray traces on the glass surface. To estimate the thicknesses of the other Cu2O films, a simple optical 207
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oxygen. The thicknesses seem to be the same for samples treated for 15, 20 and 25 s. For the sake of comparison, a thickness of 102 nm was used for calculations involving the sample treated for 30 s. Transmission spectra and thicknesses (d) were used for band gap calculation. The absorption coefficient, α, was estimated by using the expression α = ln(T)/d. Energy gaps were determined from the Taucplot, in which (α E)n is plotted against E, by intersecting the E axis with a linear fit around the fundamental absorption region. Depending on the nature of the material, different fundamental transition values are assigned to n. For indirect band gap materials, as it is the case for CuO, n has a value of 1/2. For Cu2O, the fundamental transition is direct forbidden, which has an n value of 2/3 [8]. It can be seen, from Table 1, that CuO annealed at 400 °C has a band gap of 1.20 eV, which is consistent with reported values of sputtered CuO films [19]. When CuO film-targets are plasma treated for only 15 s, a band gap of 2.34 eV is obtained. The sudden increase in band gap energy clearly shows the conversion of CuO to Cu2O by means of the plasma. As Martinez et al. explain in reference [8], band gap reported values for Cu2O largely vary, from 2.0 for sol-gel synthesis to 2.5 eV for sputtering deposition. The differences arise from contribution of several mechanisms and from Cu2O film thicknesses. Band gap values of 2.37 and 2.36 eV are calculated for tP′s of 20 and 25 s, respectively. For tP = 30 s a band gap of 2.16 eV is attained; this value is close to the one ascribed for a natural Cu2O single crystal [1]. The difference in energy gaps, between the films treated for 30 s and the samples with less time of processing, could be attributed to the effect of crystallite size. In reference [2], Chang et. al reported a threshold around 14 nm of crystal size below which band gap was about 2.35 eV; above the threshold, band gap was estimated to be 2.17 eV. This effect was also reported, by Wei-Hong Ke et. al., for very small Cu2O nanocubes, which sizes are controlled by chemical means [20]. A similar trend was also observed for the present work, though apparently at 19 nm as threshold (Fig. 3). Resistivity measurements using the four-probe method were also performed. From Table 1, it is seen that CuO film-targets annealed at 400 °C have a ρ = 1.7×102 Ω cm, a higher value (104 Ω cm) was reported in previous work for the same synthesis. However, for such study the thermal treatment of CuO was at 250 °C, which may explain the difference. Upon PT for 15 s, the resistivity increases rapidly to 21.2 × 102 Ω cm for Cu2O + Cu. With further tP, ρ decreases to 15.7 × 102 Ω cm for 20 s. This initial decline in resistivity, with growing exposure to plasma, could be owed to the growth of crystal size (Fig. 3). It is interesting that the resistivity of Cu2O + Cu films is bigger than that of the CuO target, this despite Cu2O has traces of metallic copper for 15 and 20 s of treatment. In this last regard, metallic copper might be dispersed in the film given its low presence, which does not add to conductivity. For the film treated for 25 s, in which there is not metallic copper, the lowest ρ value of 5.6 × 102 Ω cm is obtained. This might be attributed to Cu2O alone. On the other hand, though Cu2O films obtained at 30 s of treatment have the biggest crystallite size, they conversely show an increase in resistivity. This effect might be due to the highly disordered array of agglomerates, which would be causing the rise in such parameter.
Fig. 7. XRD patterns of a CuO film-target a) just after plasma treatment for 20 s, and b) 1 day exposed to open atmosphere.
of removing or adding oxygen to the treated material. In this regard, the full transformation of CuO, or Cu, to Cu2O is achieved during the PT. Nonetheless, the transformation of CuO to Cu2O can occur by a different mechanism, as evidenced in this report. All the samples presented in this work were measured at least a day after the plasma processing. The transformation of films to Cu2O starts in the PT; however, full conversion is achieved afterwards. CuO filmtargets are dark brown in color, but they become dark-gray, dark green or gray-yellow just after the PT. Films are stored in open atmosphere and after some hours or a day, it is evident they suffer a visible change of color to yellow or greenish yellow. To better illustrate the described phenomena, a CuO film-target with TA = 450 °C was treated with plasma for 25 s. For this specific case, all gases were shut down after processing; the sample was still cooled down for 15 min. XRD and transmittance-reflectance measurements were performed immediately after the PT, as well as a day later. It can be seen, from Fig. 7a), that Cu2O with a crystallite size = 17.5 ± 1.4 nm, and Cu are obtained just after the PT of the CuO filmtarget. The treated sample is left exposed to open atmosphere. According to Fig. 7b), the signal from metallic copper almost disappears after a day. Nevertheless, the counts for Cu2O increase while the crystal size remains virtually the same, 17.1 ± 0.9 nm. The XRD patterns suggest that an important proportion of metastable metallic copper is formed by the PT. This Cu easily oxidizes, at ambient temperature in the presence of oxygen, to produce crystalline Cu2O. The described changes are also observable in the transmittance-reflectance spectra of Fig. 8. In comparison to the same sample measured just after the PT, the film becomes much more transparent and less reflecting after a day. The higher transmittance can be assigned to Cu2O as the majority phase, while the lower transmittance is mainly due to reflecting metallic copper. Moreover, the sum of reflectance + transmittance is less than 40% for the film just plasma-treated. The last indicates that a high proportion of metallic copper is absorbing light, that would explain the gray or dark color observed for samples just after the plasma processing. After a day, when copper oxidizes, reflectance + transmittance increases to 75%, which means there is a lower proportion of Cu. The evidence presented suggests that Cu2O is being obtained after
4. CuO reduction phenomena by plasma treatment The conversion mechanism of CuO to Cu2O has been discussed before in literature. Plasma spectroscopy has suggested the formation of several ionized and dissociated species, which are dependent on the gases used. In the reduction of CuO to Cu2O, nitrogen and hydrogen ionized species remove oxygen from CuO by forming nitrogen oxides and water [5,10,14]. When metallic copper plates are used as a source, an applied oxygen plasma produces Cu2O and/or CuO by absorbing oxygen. Ionized and dissociated oxygen species are also formed during these treatments. Moreover, argon is commonly used along with oxygen; its role as ionized species is not well understood, however [12]. The referenced reports explain the obtaining of Cu2O as a consequence 208
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Reflectance, Transmittance (%)
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01/256399 and CeMIE-Sol-PY207450/25. The authors also thanks CONACyT for the fellowship with No. 378107 awarded to Ms. C. Miguel Ángel Badillo Ávila. Authors thank the technical assistance of Bs. C. Dinora Coria Quiñones and Saúl Velázquez Rivera. Special thanks are due to Adair Jiménez Nieto for SEM morphology measurements.
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References [1] H. Matsumoto, K. Saito, M. Hasuo, S. Kono, N. Nagasawa, Revived interest on yellow-exciton series in Cu2O: an experimental aspect, Solid State Commun. 97 (1996) 125–129. [2] Y. Chang, J.J. Teo, H.C. Zeng, Formation of colloidal CuO nanocrystallites and their spherical aggregation and reductive transformation to hollow Cu2O nanospheres, Langmuir 21 (2005) 1074–1079. [3] A.S. Zoolfakar, R.A. Rani, A.J. Morfa, A.P. O’Mullane, K. Kalantar-zadeh, Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications, J. Mater. Chem. C 2 (2014) 5247–5270. [4] Y. Bessekhouad, D. Robert, J. Weber, Bi2S3/TiO2 and CdS/TiO2 heterojunctions as an available configuration for photocatalytic degradation of organic pollutant, J. Photochem. Photobiol. A Chem. 163 (2004) 569–580. [5] R.C. Wang, H.Y. Lin, Efficient surface enhanced Raman scattering from Cu2O porous nanowires transformed from CuO nanowires by plasma treatments, Mater. Chem. Phys. 136 (2012) 661–665. [6] C.S. Tan, S.C. Hsu, W.H. Ke, L.J. Chen, M.H. Huang, Facet-dependent electrical conductivity properties of Cu2O crystals, Nano Lett. 15 (2015) 2155–2160. [7] C.-Y. Chu, M.H. Huang, Facet-dependent photocatalytic properties of Cu2O crystals probed by using electron, hole and radical scavengers, J. Mater. Chem. A5 (2017) 15116–15123. [8] G. Martinez-Saucedo, R. Castanedo-Perez, G. Torres-Delgado, A. Mendoza-Galvan, O.Z. Angel, Cuprous oxide thin films obtained by dip-coating method using rapid thermal annealing treatments, Mater. Sci. Semicond. Process. 68 (2017) 133–139. [9] C. Ooi, G.K.L. Goh, Formation of cuprous oxide films via oxygen plasma, Thin Solid Films 518 (2010) e98–e100. [10] M.J. Chen, C.-Y. Wu, Y.M. Kuo, H.Y. Chen, C.H. Tsai, Preparation of Cu2O nanowires by thermal oxidation-plasma reduction method, Appl. Phys. A 108 (2012) 133–141. [11] K.V. Rajani, S. Daniels, E. Mcglynn, R.P. Gandhiraman, R. Groarke, P.J. Mcnally, Low temperature growth technique for nanocrystalline cuprous oxide thin films using microwave plasma oxidation of copper, Mater. Lett. 71 (2012) 160–163. [12] W. Wu, et al., Characterization of Cu2O and Cu2O/Ag2O thin films synthesized by plasma oxidation, Vaccum 118 (2015) 147–151. [13] Y.M. Chan, Y.T. Wu, S. Jou, Oxide solar cells fabricated using zinc oxide and plasma-oxidized cuprous oxide, Jpn. J. Appl. Phys. 51 (2012) 125502-1–125502-4. [14] S. Han, H.Y. Chen, L.T. Kuo, C.H. Tsai, Characterization of cuprous oxide films prepared by post-annealing of cupric oxide using an atmospheric nitrogen pressure plasma torch, Thin Solid Films 517 (2008) 1195–1199. [15] H. Chen, J. Mai, Preparation and characterization of cuprous oxide thin films by an atmospheric pressure plasma annealing, in: Proceedings of the 22nd International Symposium on Plasma Chemistry, 2015, pp. 9–10. [16] J.M. Criado, M.J. Diánez, L.A.Pérez. Maqueda, Modificación de un horno doméstico de microondas para el tratamiento de materiales a alta temperatura bajo atmósfera controlada, in Congreso Conamet/SAM, 2008. [17] S. Tajima, S. Tsuchiya, M. Matsumori, S. Nakatsuka, T. Ichiki, High-rate reduction of copper oxide using atmospheric-pressure inductively coupled plasma microjets, Thin Solid Films 519 (2011) 6773–6777. [18] H.E. Bennett, J.O. Porteus, Relation between surface roughness and specular reflectance at normal incidence, J. Opt. Soc. Am. 51 (1961) 123–129. [19] S. Rehman, A. Mumtaz, S.K. Hasanain, Size effects on the magnetic and optical properties of CuO nanoparticles, J. Nanopart. Res. 13 (2011) 2497–2507. [20] W.H. Ke, C.F. Hsia, Y.J. Chen, M.H. Huang, Synthesis of ultrasmall Cu2O nanocubes and octahedra with tunable sizes for facet-dependent optical property examination, Small (2016) 3530–3534. [21] A.E. Rakhshani, Preparation, characteristics and photovoltaic properties of cuprous oxide – a review, Solid. State Electron. 29 (1986) 7–17.
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Wavelength (nm) Fig. 8. Optical spectra of a CuO film-target a) just after tP of 20 s, and b) 1 day exposed to open atmosphere.
the metastable metallic copper oxidizes. It is intriguing, though, that Cu2O instead of CuO is achieved by the copper oxidation. The last given that CuO has been reported to be the most stable oxide of copper at ambient conditions [21]. This unusual oxidation process provides an opportunity to further study the effects of after-plasma-treatment parameters. Current research is under development. 5. Conclusions In less than 30 s, Cu2O thin films were produced from an argon/dryair plasma treatment of CuO films. CuO films were prepared from solgel synthesis by dip coating, which had not been previously reported to be used in plasma systems. Conventional annealing temperatures of CuO film-targets showed that plasma-produced Cu2O crystallite sizes are bigger when lower TA′s are used. However, there was an optimal TA around 400 °C for which crystallite size was maximized to 25 nm for 30 s of treatment. Energy gap values were determined to be around 2.35 eV for Cu2O samples produced at less than 25 s. Nonetheless, for 30 s of treatment a band gap of 2.16 was found, which was practically the same that the one reported for a single Cu2O crystal. The lowest resistivity of a Cu2O film, obtained at 25 s of plasma processing, was found to be 5.6 × 102 Ω cm. Most importantly, it was proved that an argon/dry-air PT reduces CuO to a metastable form of crystalline metallic copper; in less than a day in open atmosphere, metallic copper gradually oxidizes and transforms to crystalline Cu2O. This oxidation phenomenon had not been reported before, but it could be used to tailor many of the properties of Cu2O. Acknowledgments This work was supported by Consejo Nacional de Ciencia y Tecnología (CONACyT) under projects CB-2013-01/222909, CB-2015-
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