Structural Metastability and Quantum Confinement in Zn1

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Structural metastability and quantum confinement in Zn1-xCoxO nanoparticles Gloria Almonacid, Rosa Martín-Rodríguez, Carlos Renero-Lecuna, Julio Pellicer-Porres, Saïd Agouram, Rafael Valiente, Jesus Gonzalez, Fernando Rodriguez, Lucie Nataf, Daniel R. Gamelin, and Alfredo Segura Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02230 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

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Structural metastability and quantum confinement in Zn1-xCoxO nanoparticles G. Almonacid,1 R. Martín-Rodríguez,2,3* C. Renero-Lecuna,2,4 J. Pellicer-Porres,1 S. Agouram,4 R. Valiente, 2 J. González,5 F. Rodríguez,5 L. Nataf,6 D. R. Gamelin,7 and A. Segura 1 1

ICMUV, MALTA-CONSOLIDER Team, Departamento de Física Aplicada, Universitat de

Valencia, E-46100 Burjassot (Valencia), Spain 2

Departamento de Física Aplicada, MALTA-CONSOLIDER Team, Universidad de Cantabria –

IDIVAL, Santander, E-39005, Spain 3

Departamento de Química e Ingeniería de Procesos y Recursos, ETSIIyT, Universidad de

Cantabria – IDIVAL, Santander, E-39005 Spain 4

Departamento de Física Aplicada, Universitat de Valencia, E-46100 Burjassot (Valencia),

Spain 5

DCITIMAC, MALTA-CONSOLIDER Team, Universidad de Cantabria, Santander, E-39005,

Spain 6

ODE Beamline, Synchrotron Soleil, L’Orme des Merisiers, BP48 Saint Aubin, F-91192 Gif-

sur-Yvette Cedex, France 7

Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA

1 ACS Paragon Plus Environment

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Abstract

This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1-xCoxO nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption and Transmission Electron Microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W-NPs. In contrast to the abrupt phase transition in W-Zn1-xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co2+ are observed in NPs from ambient pressure to about 17 GPa. Above this pressure, several changes in the absorption spectrum reveal a gradual and irreversible W-to-RS phase transition: i) the fundamental band-toband edge shifts to higher photon energies; ii) the charge-transfer absorption band virtually disappears (or overlaps the fundamental edge); and iii) the intensity of the crystal-field absorption peaks of Co2+ around 2 eV decreases by an order of magnitude and shifts to 2.5 eV. After incomplete phase transition pressure cycles, the absorption edge of non-transformed WNPs, at ambient pressure, exhibits a blue-shift of 0.22 eV. This extra shift is interpreted in terms of quantum confinement effects. The observed gradual phase transition and metastability are related to the NP size distribution: the larger the NP the lower the W-to-RS transition pressure.

KEYWORDS: Zn1-xCoxO nanoparticles, Phase transition, Absorption, Metastability

INTRODUCTION Fostered by predictions of ferromagnetic behavior at room temperature (RT) in wurtzite (W)-Zn11,2

xMnxO,

W-ZnO based diluted magnetic semiconductor (DMS) alloys have been intensively

investigated in the last decades. Among them, W-Zn1-xCoxO is one of the most thoroughly investigated. Zn1-xCoxO single crystals,3-5 thin films,6-11 nanowires12,13 and nanoparticles (NPs)1422

have been synthesized and their optical, transport and magnetic properties extensively studied. 2 ACS Paragon Plus Environment

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In spite of the large amount of literature on the subject, RT ferromagnetism in W-Zn1-xCoxO and its underlying mechanism remain controversial as reflected in extensive reviews on the subject.2325

Beyond magnetic properties, new applications of W-Zn1-xCoxO thin films and nanostructures are emerging in fields like photocatalysis,26-30 photoelectrochemical hydrogen production from water or hybrid solar cells.31,32 These new applications take advantage of the presence of intense absorption bands in the near infrared (NIR) and visible range, associated to substitutional Co atoms in tetrahedral coordination. These bands arise from very specific features of W-Zn1-xCoxO electronic structure, namely the electronic states associated to the Co 3d shell in a tetrahedral crystal field. The lack of inversion symmetry leads to very intense optical transition in the Co 3d shell and charge transfer transitions (CTT) between the Co 3d states and the semiconductor alloy bands. The way these absorption bands contribute to photovoltaic and photoconductivity effects is a critical issue for the above referred application and has been the object of several studies.18,33-36 High pressure techniques are well suited to investigate the electronic structure of semiconductor materials37 and have already been used to study the electronic structure of Zn1-xCoxO NPs,38 thin films,35,39-41 and single crystals.4,42 High pressure was also crucial to elucidate electronic structure effects related to quantum confinement in quantum dots.43 The present work investigates the effect of pressure on the structure and electronic properties of Zn1-xCoxO NPs. After the structural characterization by high-resolution electron microscopy and X-ray absorption, the pressure behavior of the optical absorption spectra is analyzed. Finally, we discuss some relevant features related to the pressure induced W-to-RS transition in NPs, with focus on metastability and quantum confinement effects.

Structural and compositional characterization: electron microscopy 3 ACS Paragon Plus Environment

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Figure 1. A) Low and high resolution TEM image of Zn1-xCoxO nanoparticles with 30% of Co. B) High resolution TEM image of a nanoparticle. C) Fast Fourier Transform of image B. D) Bars: Grain-size distribution histograms for pure ZnO and Zn1-xCoxO NPs (x = 0.05, 0.15 and 0.3). Lines: Gaussian fits to the bar histograms. E) XRD patterns of pure ZnO, Zn1-xCoxO (x = 0.05 and 0.3) and pure W-CoO NPs. Figures 1.A-C show a representative transmission electron microscopy (TEM) micrograph of Zn1xCoxO

NPs (x = 0.3), the sample morphology, a high resolution TEM (HRTEM) and the fast

Fourier transform (FFT) of Fig. 1.B. The sample consists of NPs with diameter ranging from 3 to 8 nm (Figure 1.D). Size distribution was also analogously determined for NPs of pure ZnO and Zn1-xCoxO (x = 0.05, 0.15, 0.30) (Figure 1.D). The X-ray diffraction (XRD) patterns of pure ZnO, Zn1-xCoxO (x = 0.05 and 0.30) and pure CoO NPs shown in Figure 1.E indicate a good crystallization of the NPs in the W structure. The cell parameters obtained for W doped-ZnO NPs, a=3.2078 Ǻ and c=5.1962 Ǻ are slightly smaller than the standard bulk values. As shown in the


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histograms of Figure 1.D, a Gaussian function was fitted to the grain size distribution through the equation:

N (d ) 

NM e 2   d

1  d d0     2   d 

2

(1)

Where d0 is the average NP diameter, d the distribution width and NM the number of NPs. Table 1 collects the fitting parameters of different Co content samples. The mean diameter (d0 ≈ 4.5 nm), which is fairly independent of the Co content, is between two and three times larger than the effective Bohr radius of the bulk exciton in ZnO (1.8 nm). Therefore, a weak confinement should be expected for average (and larger) size NPs, even at low temperature. Table 1. Gaussian parameters for size-grain-nanoparticles distribution where NM is the total number of NPs, d0 is the NPs average size and d the Gaussian FWHM Sample

NM

d0 (nm)

d (nm)

ZnO

246

5.2 ± 0.1

1.3 ± 0.1

Zn0.95Co0.05O

238

4.5 ± 0.1

1.0 ± 0.1

Zn0.85Co0.15O

224

4.6 ± 0.1

0.9 ± 0.1

Zn0.7Co0.3O

208

4.6 ± 0.1

1.0 ± 0.1

As mentioned before, Figure 1.B shows the HRTEM image of x = 0.05 Co-doped ZnO NP and Figure 1.C is the FFT. Both images suggest that each NP is a single nano-crystal. The intensity profiles through the perpendicular to a given crystallographic plane exhibit equidistant maxima. In the FFT all the intense reflections correspond to a single crystal. From the intensity profile, an interplanar distance of around 0.284 nm is obtained, corresponding to the (100) family of the Wphase (0.281 nm in bulk ZnO).44-46 Similarly, selected area electron diffraction (SAED) patterns (not shown here), show diffraction rings corresponding to the (100) and (002) reflections. All NPs are, therefore, in the W-structure at ambient conditions, with a small change in the lattice 5 ACS Paragon Plus Environment

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parameters with respect to bulk ZnO. No other structural phase was observed by either TEM or Xray diffraction (XRD) techniques.19 The measured interplanar distances, and associated lattice parameters a and c, as obtained from SAED, FFT and intensity profile are collected in Table 2. On increasing the cobalt concentration a increases and c decreases, consistently with previous thin film XRD data for low Co concentration.47 This behavior can be understood by the increase of the ionic character of the alloy as the Co concentration increases (Co 3d electrons contribute less to covalent bonds than Zn 3d electrons). Table 2. Interplanar distances and lattice parameters. Sample

d (100) Å

a (Å)

d (002) Å

c (Å)

ZnO

2.843 ± 0.007

3.283 ± 0.008

2.623 ± 0.004

5.246 ± 0.008

ZnO 5 % Co

2.883 ± 0.012

3.329 ± 0.014

2.620 ± 0.030

5.230 ± 0.060

ZnO 15 % Co

2.900 ± 0.005

3.341 ± 0.006

2.573 ± 0.002

5.146 ± 0.004

ZnO 30 % Co

2.870 ± 0.004

3.314 ± 0.005

2.572 ± 0.011

5.140 ± 0.020

Figure 2. Low resolution TEM image (A) and EDX mapping for Zn1-xCoxO NPs (x = 0.15) Co. Red = Oxygen (B); yellow = zinc (C) and green = cobalt (D).


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In order to check the compositional homogeneity of NPs, a mapping of elements was plotted by EDX for every sample. Figure 2 correlates the low magnification image of Zn0.85Co0.15O NPs to the map distribution of O, Co and Zn (in red, green and yellow, respectively). Within the experimental resolution, these maps clearly indicate that Zn and Co homogeneously distribute throughout the material and no sign of agglomeration/segregation of Co was observed. The Energy Dispersive X-ray Spectroscopy (EDX) data show that the EDX Co content is within the experimental uncertainty, very close to the nominal content (Table 3). Table 3. Actual composition of NPs by EDX analysis. Sample nominal composition

EDX Co content

EDX Zn content

(%)

(%)

Zn0.95Co0.05O

4.9 ± 0.5

95.1 ± 0.5

Zn0.85Co0.15O

11 ± 2

89 ± 2

Zn0.7Co0.3O

27 ± 5

74 ± 5

Structural and compositional characterization: X-ray absorption Figure 3 shows the XANES spectra of Zn1-xCoxO NPs for different Co content (x), normalized to an average unit absorption jump. All spectra are similar, becoming noisier for lower Co concentrations. The absorption edge, determined by the first inflection point, is located at 7720.0±0.4 eV independently of Co concentration. This value is characteristic of Co2+ oxidation state, expected in Zn-substituted Co in the W-structure,47 and also corresponds to the Co oxidation state in RS-CoO.


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Figure 3. Co K-edge XANES spectrum of Zn1-xCoxO NPs. The spectrum labeled TF corresponds to the thin film reported in Ref. 72 and the CoO spectrum from Ref. 47. The intensity and energy of XANES resonances can be used as a fingerprint to identify the structural environment of the absorbing element. There is ample literature dealing with the structural characterization of Zn1-xCoxO by means of XANES.10,20,48-57 According to electric-dipole selection rules, the pre-edge maximum, A, related to Co 1s→3d transition is forbidden if Co site has inversion symmetry (strictly forbidden in an isolated molecule or at the Brillouin zone center of a periodic solid). If the site has no inversion symmetry p-d mixing (both Co3d-O2p and Co3dCo4p) enhances the 1s→3d transition oscillator strength yielding an increase of the A intensity. So that the intensity enhances in tetrahedral Co (like substitutional Co in W-Zn1-xCoxO), whereas in octahedral Co (as in RS-CoO), p-d mixing is forbidden at the Brillouin zone and the pre-edge peak intensity is much weaker or even suppressed. The Co pre-edge in W-Zn1-xCoxO NPs (Fig. 3) shows an absorption intensity of 0.05±0.01, which is significantly smaller than that measured in W-Zn1-xCoxO thin films.46


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On the other hand, the white line (B) intensity in W-Zn1-xCoxO depends on polarization, and ranges from 1.25 to 1.75, in normalized absorption,58 while in RS-CoO, is more intense (1.8).9 The spectra corresponding to our W-Zn1-xCoxO NPs display values between 1.5 and 1.8. The set of resonances forming the white line in W-Zn1-xCoxO is complex. If no polarization is used a shoulder C is observed next to the white line. In RS-CoO,10 resonance C is slightly shifted to higher energies and the valley between B and C is more pronounced. Finally, D and E in W-Zn1xCoxO

appear as a doublet-like resonance, whereas in RS-structure E is smeared, resulting in a

triangular-like structure (not included in Fig. 3).59 All these features suggest that spectra of Fig. 3 are a mixture of tetrahedral and octahedral Co2+ spectra. Interestingly, hydrostatic pressure provides quantitative information about their relative concentration. In W-Zn1-xMnxO, remarkable changes in the XANES spectra between the W-phase and the high pressure RS-polymorph have been previously reported.59 Figure 4 shows the XANES spectra of bulk W- and RS-Zn0.75Co0.25O at 6.1 GPa in comparison to the XANES spectrum of Zn0.70Co0.30O NPs. The XANES of pure W-Zn0.75Co0.25O was taken in the pressure upstroke. At 12 GPa the transition to the RS-phase is completed. The XANES spectrum of pure RSZn0.75Co0.25O, which was taken in the pressure downstroke, indicates that the RS-phase remains metastable. XANES resonances of both phases follow the description explained above. For comparison, the spectrum of the Zn0.70Co0.30O NPs is also displayed in between. A 50% weighted spectrum corresponding to the low and high-pressure phases, reproduces the main experimental features of the NPs XANES spectrum (dotted line in Fig. 4).


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Figure 4. The Co K-edge XANES spectrum of Zn0.70Co0.30O NPs is compared to those of Zn0.75Co0.25O in the W and RS phases, both at 6.1 GPa (see text for details). The dotted line corresponds to a 50% mixture of W- and RS-spectra. We conclude that nearly 50% of Co occupies tetrahedral sites in W-Zn1-xCoxO NPs, irrespectively of x, the remainder 50% Co being in octahedral coordination. Given that it is not observed by STEM nor XRD, most octahedral Co is probably in the form of amorphous NPs. It must be noted, as we will discuss below, that all the other characterization techniques used here are not sensitive to octahedral Co, due to the local inversion symmetry which strongly reduces the intensity of optical transitions in the Co 3d shell. As elusive as the presence of octahedral Co in Zn1-xCoxO NPs might appear, it has been previously detected by Electron Paramagnetic Resonance (EPR).60

Optical absorption of Zn1-xCoxO NPs at ambient pressure 10 ACS Paragon Plus Environment

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Figure 5 shows the absorption spectra at ambient conditions of a W-Zn1-xCoxO NPs series. Absorption bands at 2 eV are due to 3d-3d* crystal-field transitions of tetrahedral Co2+, from the 4

A2(F) ground state to different excited states arising from the 3d7 configuration. These spectra are

the signature of Zn2+-substitutional Co2+ in W-ZnO,56 and have also been observed in bulk6,61 and thin films.38, 62 In particular, the observed absorption bands are assigned to transition 4A2(F)  4

T1(P) (2.02 eV), with contributions from 4A2(F)  2E(G) (1.89 eV) and 4A2(F)  2A1(G) (2.18

eV),38,63,64 their intensity being linear with Co concentration (inset of Fig. 5).65

Figure 5. Ambient pressure absorption spectra of the of Zn1-xCoxO NPs for different Co concentrations. The inset shows the absorption intensity of the most intense absorption peaks as a function of the Co content. Spectra in Figure 5 show the NP platelet optical density (O.D.), as obtained from the optical transmittance, T, through O.D.   log( T ) . Given that the average thickness of the NP platelets is 20 µm, the absorption coefficient was derived through the equation 𝛼𝑒𝑓𝑓 =

𝑂.𝐷. 𝑑

· ln(10) ≈

1150 𝑂. 𝐷. (cm−1 ). Hence the Co-concentration dependence of the effective absorption coefficient at the band maximum can be expressed as 𝛼𝑒𝑓𝑓,𝑚𝑎𝑥 (𝑥) = 2.8 · 103 𝑥 (cm−1 ). A similar empirical equation for the absorption coefficient of the Co 3d-3d* red absorption band


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(2.02 eV) was obtained from thin film data, 𝛼𝑀𝐴𝑋 (𝑥) = 8.7 · 104 𝑥 (cm−1 ).10,38 By comparing these equations, we conclude that Zn1-xCoxO NPs occupy around 3% of the platelet volume. The absorption edge corresponding to inter-band transitions must shift toward higher energies with increasing the Co concentration.19,40 Indeed, we observe this shift in the absorption spectra on passing from pure ZnO NPs to Zn0.99Co0.01O NPs. The latter sample exhibits a low energy tail associated to the CTT.18 For higher Co concentrations the absorption spectrum beyond 2.5 eV is dominated by the CTT absorption edge whose intensity increases with Co concentration, giving rise to an apparent shift to lower energy and subsequent overlap of the CTT low energy tail with Co 3d-3d* absorption bands for the highest Co concentration. Optical absorption of (Zn, Co)O NPs under high pressure Figure 6 shows the absorption edge of three samples of Zn1-xCoxO NPs (x = 0.02, 0.05 and 0.20) as a function of pressure in the 0-23 GPa range. There is a clear difference between the pressure shift of Co 3d-3d* absorption bands, and CTT and interband absorption edges. While the latter clearly shift to higher energies with pressure, Co 3d-3d* bands are less sensitive to pressure, as previously observed,32 although their intensity abruptly decreases, above 10 GPa, as a consequence of the W-to-RS phase transition. There is a tight correlation between the energy shift and intensity changes in the absorption spectra. Up to around 8-10 GPa, both interband and CTT absorption edges shift almost parallel towards higher energies, with a pressure coefficient (photon energy at constant absorbance) of 25 meV/GPa, very similar to that reported in Ref. 39 for low Co concentration in Zn1-xCoxO thin films. Absorption bands related to crystal-field Co 3d levels practically do not shift with pressure (< 1meV/GPa) but their intensity experiences a slight decrease.


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Figure 6. Pressure dependence of the absorption spectra under pressure of Zn1-xCoxO NPs with x = 0.02 (A), x = 0.05 (B) and x = 0.20 (C). At higher pressures (10-18 GPa) depending on the Co concentration, two remarkable features are observed: i) the absorption edge changes its shape, shifts towards higher energies and progressively decreases in intensity, and ii) the Co 3d-3d* band intensity strongly reduces. Both 13 ACS Paragon Plus Environment

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effects are correlated and the pressure beyond which they appear is smaller for larger Co concentrations. When O.D. ≤ 2, the shape of the CTT and the direct interband absorption edge becomes similar in NPs and thin films,39 indicating that the effective W-Zn1-xCoxO contents in the platelets is about the same order as in 150 nm thick thin films. Finally, the absorption spectrum loses all the features associated with the W-phase beyond a given pressure, suggesting a pressure-induced phase-transition, that is also larger for lower Co concentration (20 GPa for x = 0.02 versus 16 GPa for x = 0.3). Its shape changes from a steeped absorption edge in W-Zn1-xCoxO to a monotonously increasing absorption edge, shifted to higher photon energies, in RS-Zn1-xCoxO, as observed in RS-ZnO thin films.66 All these changes suggest a gradual transition from W- to RS-phase in which some NPs remain in the W-phase up to nearly 20 GPa. Given that RS-Zn1-xCoxO shows no relevant absorption feature associated with Co2+ 3d-3d* transitions, their intensity between 1.5 and 2.4 eV can be used as a local probe of W-phase NPs at a given pressure. According to the results presented in previous sections, the intensity of Co2+ 3d-3d* absorption bands is proportional to the content of Co in tetrahedral coordination. Then the ratio of Zn1-xCoxO NPs remaining in the W-phase at a given pressure (with respect to ambient pressure) has been followed through the relative change of the 3d-3d* absorption band integrated intensity as shown in Figure 7 for 5 different Co concentrations. The shape of the intensity variation, with its gradual decrease at the W-to-RS pressure-induced phase transition, can be modelled by a complementary error function:  1 A( P0 )  AMax 1   2 P 



e 0

1  P  P0   2  P

   

2

 dP  

(2)

where P0 is the central pressure at which the phase transition is half completed, P is the transition pressure “width” (pressure interval through which transition takes place) and AMax the total Co2+ integrated band intensity when the transition begins. Figure 7 also shows the fitting of the experimental data to equation (2) for each sample, the parameters of which are collected in Table 14 ACS Paragon Plus Environment

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4. The gradual W-to-RS transition in Zn1-xCoxO NPs starts at lower pressures for larger Co content (x). This trend clearly indicates that the pressure at which NPs transit from W- to RS-phase depends on the Co content as previously observed for thin films.33 On the contrary, the width of the transition seems to be independent of the Co content, although it is smaller for x = 0.1 whose size distribution was also the narrowest (see Tables 1 and 4). This observation suggests that the transition width is correlated to the NP size distribution, an issue that is discussed in the following section. Table 4. Parameters obtained by fitting Eq. 2 to the experimental data. Co nominal content

P0

P

AMax

(GPa)

(GPa)

(eV)

2

18.3 ± 0.5

1.85± 0.05

0.030

5

15.9 ± 0.5

1.72 ± 0.5

0.055

10

15.8 ± 0.4

0.88 ± 0.02

0.113

20

13.7 ± 0.5

1.35 ± 0.4

0.206

30

13.8

1.7 ± 0.4

0.130

(%)

Figure 7. Normalized integrated intensity as a function of pressure (symbols), and fits of Equation 3 to the experimental data for each sample (lines). 15 ACS Paragon Plus Environment

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Quantum confinement effects in recovered samples The fact that the smallest particles remain in the low pressure W-phase up to higher pressures has been previously observed for many semiconductors 43 and can be attributed to the stabilizing role of surface energy. Changes in the absorption spectra observed near to the W-to-RS transition are, most likely, related to the size of non-converted NPs. To further investigate the possible correlation between size and transition pressure, optical absorption measurements under pressure were carried out up to the pressure at which the absorption spectra of certain NPs still maintains features of the W-structure, i.e. before the transition to the RS-phase is fully completed. At this pressure the absorption edge of Zn1-xCoxO NPs is similar to that of W-Zn1-xCoxO thin films.39 This is the fingerprint of NPs still remaining in the W-phase (see for instance spectrum at 17 GPa of Fig. 6-B). Under these conditions the absorption spectrum is dominated by W-phase NPs, since the optical bandgap for the RS-NPs is shifted to much higher energies and the octahedral Co2+ d-d* band intensity in the RS-phase is negligible (see spectra at 22 GPa in Fig. 6-A, B). The variation of the absorption spectra of Zn0.95Co0,05O NPs in the upstroke and downstroke (Figure 8) reveals quantum confinement effects. As expected, the NP bandgap shifts to lower energies when pressure decreases in down-stroke. However, once the pressure cycle is completed, the absorption spectrum at ambient pressure appears shifted to higher photon energies with respect to the spectrum before the pressure cycle. To avoid apparent pressure shifts associated with different sample thicknesses, Figure 9 compares the ambient pressure spectrum of Fig 7-B with the one of a thin film with the same Co content (x = 0.05).39,40 The spectrum of the recovered NP sample is shifted by 200 meV to higher energies, reflecting quantum confinement effects.


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Figure 8. Pressure dependence of the absorption spectra of Zn1-xCoxO NPs with 5% Co in upstroke (A) and downstroke (B). The size of NPs with W-phase in the recovered sample can be estimated from the absorption blue shift of 200 meV on the assumption of spherical NPs. In such a case, the confinement energy for the interband transition involving confined discrete states in the valence band and conduction bands is given by:67-70  2  n  1.8 e2 En     2 *  2 R  40  R 2

(3)

µ* is the reduced electron-hole mass and R the NP radius. Using the reduced effective mass of WZnO, a blue shift of 200 meV corresponds to NPs of 4.7 nm diameter. This size estimation is fully consistent with previous results on size dependent blue shift in the absorption of ZnO NPs.67-69 17 ACS Paragon Plus Environment

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Figure 9. Absorption spectra at ambient pressure of a recovered sample of Zn1-xCoxO NPs, x = 0.05, after a pressure cycle, and a Zn1-xCoxO thin film with the same Co content. Although the coincidence between estimated and measured NP mean size obtained from electron microscopy can be surprising at first sight, it is however consistent with the present size distribution (Fig. 1). Given that the NP volume scales with R3, NPs with larger size (in which confinement effects are negligible) do, in fact, contain most of the sample mass and thus dominate the absorption spectrum of as-prepared samples. Following results of Fig. 7, around 30% of tetrahedral Co has transited to the RS-phase at 15 GPa. According to the size distribution given in Table 2, and assuming that the larger NPs transited to the RS-phase, the maximum size of nontransited W-phase NPs would now be 5.3 nm. The effective diameter probed by optical absorption would be slightly smaller (4.6 nm) and fully consistent with the value (4.7 nm) obtained from the absorption edge blue-shift through Eq. (3). Metastable Zn1-xCoxO rock-salt NPs According to previous works on Zn1-xCoxO thin films39 and microcrystalline powders,42 the W- to RS-phase transition in bulk materials and thin films occurs at lower pressures for larger Co content. It decreases from 9 GPa in pure ZnO to 6 GPa in Zn0.7Co0.3O. The transition is reversible only for Co concentrations below 27%.39 In NPs, pressure release after complete transition to the RS-phase 18 ACS Paragon Plus Environment

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(Figure 10 A-B) causes the expected redshift of the absorption edge. However, the absorption spectrum at ambient pressure does not recover any of the specific features of the W-phase. This indicates that the RS-phase is metastable at ambient pressure regardless of the Co content, even for pure ZnO as previously reported in Ref. 71.

Figure 10. Pressure dependence of the absorption spectra of RS-Zn1-xCoxO NPs with x = 0.02 (A) and x = 0.20 (B), obtained in downstroke.

For pure ZnO and Zn1-xCoxO NPs (with x < 0.15) the absorption edge of the RS-phase is steep and intense, consistently with the assignment to a direct transition at the  point, as proposed in Ref. 40. However, for higher Co concentration (x > 0.2) it becomes less steep and extends down to 2 eV, indicating that in RS-Zn1-xCoxO there are also Co2+ 3d-3d* and CTT absorption bands. The 19 ACS Paragon Plus Environment

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inset of Figure 10-B shows the contribution of an absorption band centered around 2.4 eV, very close to the energy of the absorption bands found in RS-CoO,72 that were ascribed Co2+ 3d-3d* in octahedral coordination. It should be noted the Co2+ 3d-3d* transitions in the RS-phase are much less intense than in the W-phase, due to the inversion symmetry, and shifted to higher energies.72

Figure 11. High resolution images (A, B), FFT (C) and intensity profile (D) of metastable RSZn1-xCoxO NPs, x = 0.05. The metastable RS-phase is also confirmed by high resolution TEM in recovered samples of Zn1xCoxO

NPs (x = 0.05) after a pressure cycle (Fig. 11). As low and high resolution images show

(Fig. 11-A, B), recovered NPs tend to aggregate in 10 nm size NPs. Clear interplanar distances of 2.50 and 2.16 Å are visible in Fig. 11-B, indicating highly crystalline NPs. These values are very 20 ACS Paragon Plus Environment

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close to the (111) and (200) interplanar distances in RS-ZnO (2.47 Å and 2.14 Å, respectively).7173

Fig. 11-C and -D shows the FFT pattern along the [0-11] axis and intensity profile of the selected

particle which can be clearly indexed as RS-ZnO.

CONCLUSIONS We have characterized the structure, composition and electronic structure of synthesized Zn1xCoxO

NPs with monomodal size distribution (centered around 4-5 nm, independently of the Co

content). We show that the W-phase lattice parameters of Zn1-xCoxO NPs are similar to those found in Zn1-xCoxO thin films, with a slight increase of the a and decrease of the c parameters as the Co concentration increases. In agreement with previous EPR data, XAS reveals that about 50% of Co atoms are in octahedral coordination, probably in the form of amorphous NPs, not detected by either optical measurements, XRD or STEM. We demonstrate throughout optical absorption measurements under pressure new insights on the W-to-RS phase transition mechanism and the metastable character of the RS-phase in Zn1-xCoxO NPs. In particular, we have showed that the Wto-RS phase transition is gradual, the transition pressure being inversely correlated with the NP size. The W-phase remains stable up to nearly 20 GPa in the smallest NPs (d < 4.7 nm). In recovered samples after downstroke, we demonstrated that the blue-shift of the absorption edge and CTT absorption band exhibited by non-transited W-phase NPs are due to quantum confinement effects. NPs converted to the RS-phase remain metastable at ambient pressure independent of the size and Co content.

EXPERIMENTAL SECTION Pure ZnO and Zn1-xCoxO with x < 30% were prepared using hydrolysis and condensation of acetates in dimethylsulfoxide.

More details about the experimental procedure were given 21 ACS Paragon Plus Environment

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elsewhere.14-19 NPs in colloidal suspension were stabilized by capping them with trioctylphosphine oxide.14 TEM, HRTEM, SAED and EDX experiments were conducted using a Field Emission Gun (FEG) TECNAI G2 F20 microscope operated at 200 kV (microscopy facility at SCSIE, University of Valencia). Few drops of the sonicated NPs suspension in ethanol were deposited on carbon grids to obtain the thin samples required for TEM experiments.74-75 XRD patterns were obtained using a Philips 1700 diffractometer with a Bragg-Brentanno Θ-2Θ geometry. X-ray Absorption Spectroscopy (XAS) measurements were done in the ODE beamline at Soleil Synchrotron, using an energy dispersive setup for XAS.76 The main optical device is a dynamically bent Si crystal, which in our experiment was tuned around the Co K-edge. The bent Si crystal also focused the x-ray beam into a spot down to 8080 μm2. X-ray energy calibration was accomplished with a metallic Co foil. A grazing mirror between sample and detector was used to remove harmonics. In dispersive XAS sample homogeneity is of primary importance. Bearing this in mind we compressed dried NPs between the two anvils of a membrane-type diamond anvil cell (MDAC).40 We obtained several micrometer thick pellets, with typical absorption jumps ∆(μd) ranging from 0.02 to 0.1, depending on the Co concentration, which varies the absorption coefficient (μ) and pellet thickness (d). For optical measurements, samples were obtained by laying several drops of a NP suspension in toluene, on a slide. After toluene evaporation, surfactant coated NPs form a flat aggregate from which platelets of 100-150 µm size and 10 to 20 µm thickness were obtained. High pressure optical absorption measurements in the UV-Vis-NIR range, were performed placing a NP platelet together with a 10 µm ruby microsphere into a 200 µm diameter hole drilled on a 50 µm thick steel gasket mounted in a MDAC.58 Methanol-ethanol-water (16:3:1) was used as a pressure-transmitting medium, and the pressure was determined from the ruby luminescence.77 The optical setup consists of a deuterium or tungsten lamp, fused silica lenses, reflecting 22 ACS Paragon Plus Environment

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objectives, and an UV-Vis-NIR spectrometer, which allows for transmission measurements up to the absorption edge of IIA diamonds (about 5.5 eV).66

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS The authors thank the Spanish Ministerio de Economía y Competitividad and FEDER under projects MAT2015-69508-P, MAT2012-38664-C02-01/02, and CSD2007-00045 (MALTACONSOLIDER INGENIO 2010). Financial support from the US National Science Foundation (CHE-1213283 to D.R. Gamelin) and the Generalitat Valenciana (Grant PROMETEO/2009/074 to A. Segura) is also gratefully acknowledged.

ABREVIATIONS wurtzite (W) rock-salt (RS) nanoparticles (NPs) room temperature (RT) diluted magnetic semiconductor (DMS) 23 ACS Paragon Plus Environment

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near infrared (NIR) charge transfer transitions (CTT) high resolution transmission electron microscopy (HRTEM) fast Fourier transform (FFT) selected area electron diffraction (SAED) X-ray diffraction (XRD) Energy Dispersive X-ray Spectroscopy (EDX) Electron Paramagnetic Resonance (EPR) optical density (O.D.) X-ray Absorption Spectroscopy (XAS) membrane-type diamond anvil cell (MDAC)

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Nano Letters


Nano Letters

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Structural Metastability and Quantum Confinement in Zn1

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