Infiltrated Zinc Oxide in Poly(methyl methacrylate): An Atomic Cycle

Article pubs.acs.org/JPCC

Infiltrated Zinc Oxide in Poly(methyl methacrylate): An Atomic Cycle Growth Study Leonidas E. Ocola,*,† Aine Connolly,‡ David J. Gosztola,† Richard D. Schaller,† and Angel Yanguas-Gil† †

Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States Vassar College, 124 Raymond Avenure, Poughkeepsie, New York 12604, United States

‡

S Supporting Information *

ABSTRACT: We have investigated the growth of zinc oxide in a polymer matrix by sequential infiltration synthesis (SiS). The atomic cycle-by-cycle self-terminating reaction growth investigation was done using photoluminescence (PL), Raman, and X-ray photoemission spectroscopy (XPS). Results show clear differences between Zn atom configurations at the initial stages of growth. Mono Zn atoms (O−Zn and O−Zn−O) exhibit pure UV emission with little evidence of deep level oxygen vacancy states (VO). Dimer Zn atoms (O−Zn−O−Zn and O−Zn−O−Zn−O) show strong UV and visible PL emission from VO states 20 times greater than that from the mono Zn atom configuration. After three precursor cycles, the PL emission intensity drops significantly exhibiting first evidence of crystal formation as observed with Raman spectroscopy via the presence of longitudinal optical phonons. We also report a first confirmation of energy transfer between polymer and ZnO where the polymer absorbs light at 241 nm and emits at 360 nm, which coincides with the ZnO UV emission peak. Our work shows that ZnO dimers are unique ZnO configurations with high PL intensity, unique O1s oxidation states, and sub-10 ps absorption and decay, which are interesting properties for novel quantum material applications.

M

ZnO increases light scattering and opaqueness of the composite, detrimental for the vis-transparent UV absorber applications.12 Atomic layer deposition (ALD) is a relatively recent technique used to grow semiconductor oxides such as zinc oxide (ZnO) for diverse applications.13 An ALD tool can be used to grow ZnO inside a polymer matrix14 by means of an alternate method named sequential infiltration synthesis (SiS).15,16 The SiS method utilizes similar concepts of ALD with the significant difference in process exposure times, pressure, and purpose. The purpose is to allow the precursor gases to infiltrate the polymer matrix and have the reaction to occur there. Another characteristic of SiS is that it operates at low temperatures that are below the glass transition temperature of polymers.17 Although initially the first material synthesized on polymers with low temperature ALD was Al2O3,17 there are other materials that can be synthesized at temperatures amenable with polymers, such as TiO2,19 ZnO,18−20 SnO,21 and SiO2.22 For these reasons, we have investigated using SiS as an alternate method to incorporate ZnO and other oxides inside the polymer matrix. The SiS process has a significant advantage

etal oxides, such as zinc oxide, have been extensively studied, as they find applications as components in resistive type gas sensors,1 optoelectronic devices,2,3 photocatalysis,4,5 and water treatment,6,7 just to name a few. Zinc oxide used in these applications takes the form of thin films, nanoparticles, or nanorods which are either crystalline or polycrystalline and are grown via physical deposition or chemical synthesis and have been extensively characterized. In this paper, a different form of zinc oxide, grown inside a polymer film, is studied. For many years, there have been efforts to incorporate zinc oxide (ZnO) inside poly(methyl methacrylate) (PMMA), in the form of nanoparticles or quantum dots, with the purpose of combining their optical properties to develop new applications such as UV−vis absorbers,8 nonlinear optics,9 photovoltaic applications,10 and gas sensors.11 A combination of a transparent matrix and UV absorbing ZnO would be suitable for UV-shielding materials and filters and can bring about functionalities suitable for luminescent films, displays, lightemitting diodes, photodetectors, and other optoelectronic coatings. However, synthesis of an oxide/polymer composite such as ZnO/PMMA with adequate properties is, in principle, a challenging task. Hydrophilic ZnO and hydrophobic PMMA do not blend easily and, as a rule, produce aggregation and clustering of the nanocrystalline filler. Such agglomeration of © XXXX American Chemical Society

Received: August 8, 2016 Revised: December 8, 2016 Published: December 8, 2016 A

DOI: 10.1021/acs.jpcc.6b08007 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Cross-sectional EDS data of SiS ZnO in PMMA films. (a) SEM micrograph of a PMMA coated silicon sample treated with 12 cycles of SiS ZnO and cleaved. The yellow line corresponds to the Zn L-shell X-rays originating from the electron beam trace shown by the arrow. The blue line corresponds to the Al K-shell X-rays. (b) EDS Zn L-shell X-ray data for samples treated with 1, 2, 4, and 6 cycles of SiS ZnO. The scan position is arranged to reflect the same scan position as in part a. Data has been offset for viewing clarity.

Figure 2. 3D PL data as a function of ZnO growth. Graphs plot PL intensity as a function of emission wavelength (300−525 nm) for a series of excitation wavelengths (220−285 nm): (a) Water terminated Al2O3 seed layer, first half cycle of H2O, (b) first half cycle of DEZ, (c) second half cycle of H2O, (d) second half cycle of DEZ. Contour levels represent 5000 cps/μA increments. Stick schematics illustrate the stage of ZnO growth that corresponds to each half cycle.

A key feature of an ALD process is the ability to add an atomic layer at a time. This characteristic allows for a detailed study of the formation of ZnO in the polymer matrix after each atomic step is formed. The two precursors used in the synthesis of SiS ZnO can be considered linear molecules. Water (H2O) can be represented as H−O−H, while diethylzinc (DEZ) can be represented as C2H5−Zn−C2H5. The DEZ and water reactions can be represented as

over prior efforts to incorporate ZnO in polymers in terms of superior homogeneous incorporation. Recent transmission electron microscope tomography work on SiS Al2O3 in PMMA shows that the oxide appears uniformly distributed throughout the polymer matrix and is not localized as single nanoparticles.23 It can be considered that, during the introduction of the different precursor gases used to synthesize ZnO, each is coating the inside free volume walls of the polymer matrix. Therefore, a polymer that is treated with the SiS process will have no scattering or opaqueness due to nanoparticle−polymer interfaces as in the case of the chemically synthesized processes described before. In addition, because ZnO is conductive, infiltrated ZnO in PMMA makes the infiltrated region conductive and modifies its index of refraction. As PMMA is a patternable polymer, it would be quite easy to create a myriad of structures by lithography and then afterward modifying them via SiS ZnO to create structures of interest for sensors or photonics, for example.

DEZ step: surface−OH + C2H5−Zn−C2H5 → surface−O−Zn−C2H5 + C2H6

H2O step: surface−O−Zn−C2H5 + H 2O → surface−O−Zn−OH + C2H6

Therefore, growth for ZnO is limited to adding one atom at a time during growth via SiS as only one bond can be formed at a B


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Figure 3. Raman spectra of ZnO SiS treated PMMA as a function of the number of half cycles. The spectra are offset for clarity. The figure insets correspond to images 3a, 3b, 3c, and 3d. LO = longitudinal optical phonon.

region where ZnO is active, which enables then a detailed study of the PL emission of ZnO as it is grown by the SiS process. The PL data, shown in Figure 2 as contour plots, corresponds to the emission intensity acquired in the range from 300 to 525 nm wavelengths for incident light in the range from 220 to 285 nm wavelengths. To the right of each plot is a stick schematic illustrating the growth of the ZnO crystal for each precursor half cycle assuming a wurtzite crystalline structure. Hydrogen and ethane molecules have been left out of the stick schematics for clarity. The data in Figure 2 starts with the PL map of a 2-cycle Al2O3 seed layer, terminated with a H2O half cycle (Figure 2a). The emission exhibits a peak at 348 nm at an excitation of 268 nm. After adding a single half cycle of DEZ (Figure 2b), the center of PL red-shifts in both excitation and emission due to the lower band gap of ZnO compared to Al2O3 of 8.8 eV.30 We believe this PL peak corresponds to the O−Zn bond. The new peak is located at 360 nm emission at an excitation of 274 nm. After adding a second half cycle of H2O (Figure 2c), an increase in intensity of the original O−Zn PL is observed with no shift, and an additional peak is observed at 360 nm emission (same as the O−Zn emission) and at 241 nm excitation. It has been suggested that this extra peak is due to de-excitation of free excitons, generated by absorption of the incident light by ester and carboxylic groups in PMMA, through energy transfer to the ZnO.12 This is the first time a direct observation of both peaks is shown in one single plot. The fact that absorption at 238 nm is emitted at 360 nm, which is the same UV emission wavelength for ZnO, corroborates that there is energy transfer between the PMMA and the synthesized ZnO. Finally, in Figure 2d, after a second DEZ half cycle, the PL intensity increases by over an order of magnitude and a VO emission is apparent in the visible. This implies that it is sufficient to have just two Zn atoms in a ZnO nanocrystal to observe oxygen vacancy defects, VO. In addition, as in Figure 2c, the two emission peaks at 360 nm are also observed and are more intense. (Emission intensity line scans across the maxima for each 2D plot are illustrated in Figure S5, Supporting Information, to highlight the significant increase of emission intensity when moving from one Zn atom to two.) Interpretation of the PL data is that the early growth stages of SiS ZnO in PMMA start as isolated nucleation sites that grow until the percolation limit is reached.17 To further

time for each growth step. Aluminum oxide, for example, is different, as the precursor trimethylaluminum (TMA) has three bonds to the aluminum atom, and therefore, each cycle allows one or two bonds to form for each growth step.

1. RESULTS The infiltration depth for the SiS process for ZnO in PMMA was determined by measuring the depth profile of ZnO via energy-dispersive X-ray spectroscopy (EDS).24 The data reported for PMMA shows an X-ray Zn L-shell signal detectable 300 nm deep,24 and down to other depths depending on the polymer (Supporting Information Figure S1), which demonstrates that the SiS process is not a coating process like ALD but that the precursors clearly diffuse and react deep into the polymer matrix. The data in Figure S1 also shows that the SiS process does not depend on a specific molecular structure. The EDS data, Figure 1, shows that the ZnO has not fully infiltrated the polymer film and that the distribution of ZnO in the polymer matrix is uniform through at least a couple hundred nanometers into the polymer for each growth step (Figure 1b). This allows enough reaction sites throughout the penetration depth in the resist that can be detected using standard characterization techniques such as PL, Raman spectroscopy, and XPS. This paper reports on data acquired using these three techniques on samples prepared with an incremental single precursor half cycle starting from the base Al2O3 adhesion layer up to 12 cycles of [H2O:DEZ]. We have verified that the ZnO is fully reacted under the process conditions used (Supporting Information Figure S2) where we compare our standard process with one where we increase the purge time between ZnO precursors by a factor of 10. In both cases, the infiltration depth is almost identical. PL from SiS ZnO in PMMA has been reported previously by the authors.24 PMMA is a polymer optimal for PL with ZnO studies because it has a band gap larger than that of ZnO. The reported band gap for PMMA can range from 3.825 to 4.9 eV,26 while the value reported for bulk ZnO ranges from 3.227 to 3.3 eV.28,29 Our work shows that the bandgap for ZnO on quartz is 3.35 eV and that for SiS ZnO in PMMA is 3.23 eV (Figures S3 and S4 in the Supporting Information). This difference in band gap means PMMA has minimal PL background in the spectral C


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cycle and 1.5 cycles), the total emission is small with dominant UV emission. For Zn dimers (2 cycles and 2.5 cycles), the total emission is very large and as intense as 12 cycles. There is a transition regime for Zn trimers and up to 5 cycles where the PL emission unexpectedly decreases. From 6 cycles and above, the emission increases monotonically with the number of cycles, indicating film formation. As indicated earlier, there is a clear indication that between 2.5 cycles and 4.5 cycles the visible emission is more intense than the UV emission. The source of this inversion is not well understood. It has been reported that a greater intensity in visible emission can be interpreted as an indicator of larger amounts of oxygen vacancies in the material.34 This does not explain why the UV intensity drops so significantly from Zn dimers to Zn trimers. On the other hand, it could be that the real question is why Zn dimers are so intense. If Zn dimer emission is excluded from the data in Figure 5, a clear monotonic trend of increased UV emission as a function of ZnO growth is observed. It is of interest to note that, in Figure 5b, the total areas corresponding to the UV and visible emissions after 8 cycles are the same. This implies that the same amount of photons is being emitted from both the band gap and defect states. Further analysis of the PL data is shown in Figure 6. In Figure 6a, the energy for each emission component of each spectra for a particular growth step is plotted. The data starts with the seed layer of Al2O3 terminated with a H2O step (cycle 0.5). As can be seen, there are at least four distinct emission regions for ZnO during growth. Monomer Zn atoms in cycles 1 and 1.5 (O−Zn and O−Zn−O) show UV emission around 3.35 and 3.4 eV, and only deep level defect emission around 2.45 eV. Dimer Zn atoms in cycles 2 and 2.5 (O−Zn−O−Zn and O−Zn−O−Zn−O) up to 5 cycles of [H2O:DEZ] exhibit similar components (two or three UV and one visible) with the visible component trending to lower energies. The main difference is the intensity of the UV and visible components, as illustrated in Figure 5. The final region is after 6 cycles where there is a clear trend toward a “bulk-like” emission pattern. This too is seen with the UV and visible intensities (and areas) shown in Figure 5. Actual values of assigned defect emission energies and the fit components used in Figures 4 and 6 are listed in the Supporting Information, Tables S1 and S2. In Figure 6b, all measured components of the PL spectra are assigned to known defect emission energies found in the literature.2,35−39 It is very apparent that Zn dimers and trimers exhibit emissions of Zni to valence band transitions not found elsewhere in the growth chart. This corresponds to the unusual cases where the visible emission is much larger than the UV emission in the PL spectra shown in Figure 5. Additional structural analysis was performed using Zn K-edge EXAFS of 1.2 mm thick PMMA treated with 12 cycles of SiS ZnO compared to thick ZnO film deposited by ALD, Figure 7. The X-ray data points at a diffused distribution of ZnO. Both SiS ZnO and ALD ZnO exhibit the same number of four oxygen nearest neighbors clearly demonstrating the formation of ZnO using SiS. The second nearest neighbors for SiS ZnO are one-third that of ALD ZnO. This suggests that SiS ZnO is a more diffuse or distorted form of wurtzite ZnO, unlike the standard crystalline ZnO mostly known to the metal oxide community. After the PL and Raman spectroscopy analysis, the same samples were characterized using X-ray photoemission spectroscopy (XPS). All data was calibrated to a C1s binding energy of 284.6 eV.34,40 This is possible, as it has been shown that

corroborate this hypothesis, we performed Raman spectroscopy on the same samples studied in Figure 2 along with others at higher growth cycles. Data is shown in Figure 3. Using 325 nm excitation, the longitudinal phonon spectra of ZnO were analyzed as a function of growth. Given that phonons require long-range ordering to be supported, it would be a good indicator at what point during growth initial crystalline structure would be reached. To our surprise, we find that the water terminated Al2O3 seed layer exhibits a strong phonon presence which is later completely disrupted by the first DEZ half cycle. Such disruption is maintained until after three complete [H2O:DEZ] cycles, where phonons reappear. This data was repeated several times on samples of different thicknesses to make sure this phenomenon was reproducible. The positions of the peaks match with those of ZnO nanoparticles published in the literature.31−33 The PL data underwent further analysis by curve fitting the data for each growth step. PL 3D spectra were analyzed, extracting the emission spectra at the excitation energy with maximum emission intensity. Examples of such data are shown in Figure 4 to illustrate the change in emission as a function of

Figure 4. Examples of emission spectra scaled to the area for 12 cycles to highlight the change in the emission spectra components as a function of ZnO growth (area scaling factors shown on the right side). Curves shown are the PL data and Gaussian−Lorentzian fit components plotted as a function of PL emission energy.

growth. The plots correspond to the first three cycles and then a “bulk”-like state at 12 cycles. The data is plotted as a function of emission energy instead of wavelength so as to better relate to results published in the literature. As can be observed, there are dramatic changes in the emission in the first three cycles. At one cycle, Zn monomers, there is only dominant UV emission. This changes dramatically as two or three Zn atoms are added at 2 cycles and 3 cycles, respectively. At 2 cycles, the visible emission is of similar intensity as the UV emission, and then, at 3 cycles, it is dominant. Eventually this trend reverses at 5 cycles, as seen in Figure 5. The maximum PL intensity of UV and visible (Vis) emissions and the total area under UV and visible (Vis) emission curve fits, as a function of growth cycles of SiS ZnO, are shown in Figure 5a and b. The data indicates that there are different environments during growth. For Zn monomers (1 D


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Figure 5. PL intensity and area data as a function of growth cycles of SiS ZnO. (a) Maximum intensity of UV and visible (Vis) emissions. (b) Total area under UV and visible (Vis) emission curve fits. A whole number cycle is an integer number of [H2O:DEZ] cycles, i.e., N × [H2O:DEZ]. A 0.5 cycle includes an extra H2O half cycle termination, i.e., N × [H2O:DEZ] + H2O.

under SiS conditions there is no covalent reaction between the precursors used in SiS and the polymer matrix.41,42 The only observed reaction found in the literature was reported after very long exposure times (∼30 min) of TMA with PMMA. As has been reported in this paper, exposure times were at most 4 min per cycle. In addition, the number of cycles is relatively low and the Zn atomic loading has been measured to be less than 10% from the EDS data. Four collection channels were used to measure the binding energies of Zn2p 3/2, O1s, C1s, L3 M4,5M4,5 Auger, L2M4,5M4,5 Zn Auger, Zn3s, Zn3d, and Zn3p electrons. The fourth channel also was able to detect Al2s, Al2p, and O2s emitted electrons from 1 cycle to 3 cycles. As the SiS ZnO growth progressed, these peaks disappeared. An example of the collected data for a sample with 1 cycle of [H2O:DEZ] is shown in Figure S6 (Supporting Information). The first set of XPS data analyzed was the trends in shifts of the binding energies of the electrons from the Zn atoms in the SiS ZnO as a function of growth, Figure 8. The data is plotted as a shift with respect to a nominal value found in the literature.34,40,43−47 The actual data along with the values used as reference are found in the Supporting Information, Table S3. The trends are similar to those found with the PL data: a transition from monomer Zn atoms (1 cycle) to dimer Zn atoms (2 cycles) to finally a convergence toward a “bulk”-like state. This is more apparent with the outer shell Zn3s, Zn3d, and Zn3p electrons. The data for the Zn2p 3/2 binding energy is plotted along with the kinetic energies of the L3 M4,5M4,5 and L2 M4,5M4,5 Auger electrons, as Wagner plots48,49 in Figure 9. Wagner plots are meant to be tools for the identification of chemical states of atoms in molecules or solids. Wagner plots also contain information about the Auger parameter, defined as the sum of the kinetic energy of a core−core−core Auger line and the binding energy of a core electron. The two are related by the work function of the material, which was estimated to be a constant of 4.0 eV. The modified Auger parameters for the L3 M4,5M4,5 Auger electron experimental values obtained in this work (2010.4 ± 0.3 eV) match well with experimental data reported elsewhere (2010.1 ± 0.4 eV).40,50

The modified Auger parameter data for Zn L3 M4,5M4,5 and L2 M4,5M4,5 Auger electrons exhibit a linear trend of evolution as a function of SiS ZnO growth. The slope for the L3 M4,5M4,5 Auger electrons is −2.27, and that for the L2 M4,5M4,5 Auger electrons is −3.07. The actual fitting parameters are yL3MM = −2.27x + 3304.4 and yL2MM = −3.07x + 4146.3 with R2 = 0.87 and 0.93, respectively. As can be observed in Figure 9, the modified Auger parameter is at its lowest at the initial stages (1−2 cycles) of SiS ZnO growth. It later evolves through a transition region (3−4 cycles) before settling around the bulk value for ZnO (6−12 cycles). This means SiS ZnO trends from nonstoichiometric toward stoichiometric ZnO during growth. The literature reports that a lower Auger parameter implies a higher oxidation state of Zn, while a larger value of the Auger parameter implies Zn is more metallic.49,50 This interpretation is further corroborated when analyzing the XPS data from the O1s electrons. The XPS data for O1s electrons was fitted to a set of Gaussians and Lorentzian examples shown in Figure 10 (and more data shown in Figures S7 and S8 in the Supporting Information). Figure 10 illustrates a trend of the XPS O1S spectra as a function of SiS ZnO growth in PMMA where there is a transition from a high binding energy state toward a low binding energy state that resembles bulk ZnO.34,44,51 The dashed line at 532 eV is a guide to the eye to highlight the change from before 2 cycles and after 2 cycles. The fit data shows that there are four O1s types of states during growth. The initial state at 1 cycle [H2O:DEZ] shows two components centered at around 533.5 eV and another at 532.0 eV. As an extra cycle is added (2 cycles [H2O:DEZ]), the 533.5 eV component disappears, leaving only the 532.0 eV component. At three cycles of [H2O:DEZ], a new component appears at 530.6 eV which later becomes the dominant feature at 12 cycles. The three components identified have been reported in the literature.34,40,43,44,51−56 All components have been tabulated, and the average value of each has been compared to the average values reported in the literature in Table 1, with good agreement. The first component, at 533.5 eV, is labeled Oc.34 This peak is associated with the presence of chemisorbed or dissociated O2, hydroxyl groups on the surface of ZnO such as −OH or E


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Figure 6. PL emission energies as a function of growth cycles of SiS ZnO. (a) Emission energies associated with growth regions. Large squares highlight the most intense UV emission component, while large circles highlight the most intense visible emission component. (b) Association of PL emission components to known defects in ZnO found in the literature.2,35−39

et al.52,53 They find conclusively that exposure of a clean O− ZnO surface to H2O is found to lead to the formation of the weak, distinct shoulder on the high-energy side of the O1s line, at 533.7 eV, which matches well with our result of 533.5 ± 0.8 eV. The Oc peak is present only with the 1 cycle and 1.5 cycle samples, i.e., where only isolated O−Zn− or O−Zn−O bonds have been formed, as illustrated in Figure 9. This also correlates to the deep level oxygen vacancy PL emission at 2.47 eV. The second component, at 532.0 eV, is labeled Ob.34 This peak is associated with O2− ions located in oxygen deficient regions (i.e., those around Zni and VO), whose intensity may be associated with the concentration of oxygen vacancies.34,44,51,55 However, the modified Auger parameter value appears to contradict the presence of O defect sites.40 The PL data exhibits emission at energies typically found in the same energy range as the bulk samples at 2.3 and 2.36 eV. The PL emission intensity though is over 20 times higher than that for the 1 cycle samples.

Figure 7. Zn K-edge EXAFS of 1.2 mm thick PMMA treated with 12 cycles of SiS ZnO compared to thick ZnO film deposited by ALD. First oxygen (O) and second zinc (Zn−Zn) nearest neighbors are indicated.

−CO3, adsorbed H2O, or adsorbed O2.34,43,44,51,52 The interaction of ZnO with water was studied in detail by Kunat F


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Figure 8. Binding energies of XPS emitted electrons from Zn atoms in SiS ZnO as a function of growth cycles.

Figure 9. Wagner plots of Zn2p 3/2 binding energies vs Zn L3 M4,5M4,5 and Zn L2 M4,5M4,5 Auger electron kinetic energies. Bulk data from references cited.40,43−45 The dotted line among the connecting data points is a linear fit. The dash-and-dot line corresponds to modified Auger parameters.

Therefore, the Ob peak at 2 cycles and 2.5 cycles seems to correspond to a highly oxidized state of ZnO, as shown in Figure 9 and not necessarily due to an abundance of O defect sites. Observing Figure 9, it is reasonable to suggest that the Zn−O−Zn bond, while not wurtzite yet, has unique quantum states that need further investigation. The third component, at 530.6 eV, is labeled Oa.34 This component starts to appear after 3 cycles of [H2O:DEZ] and later on dominates as the growth of SiS ZnO continues. This peak is associated with O2− ions in the wurtzite structure51 of the hexagonal Zn2+ ion array, which are surrounded by zinc atoms with the full supplement of nearest neighbor O2− ions. Therefore, the Oa peak can be attributed to the Zn−O bonds in a ZnO wurtzite crystal.34 The intensity of the Oa peak is an indication of the oxidized stoichiometric surrounding of the oxygen atoms.40,44,54,55 When the intensity of the Oa peak exceeds that of the Ob peak, there is an indication of strong Zn−O bonding in the ZnO crystal or thin film.34,43 The fact that this peak appears only after 3 cycles of [H2O:DEZ] correlates very well with the Raman data shown in Figure 3, where longitudinal optical phonons first appear in the growth of SiS ZnO. The XPS O1s ratio of the areas of Oc and Oa with respect to the area corresponding to the Ob state (Oc/Ob and Oa/Ob) was plotted as a function of growth (Figure S9, Supporting

Figure 10. XPS data for O 1s electrons fitted to a set of Gaussians and Lorentzians for samples with 1, 2, 3, and 12 cycles of [H2O:DEZ]. The dashed line at 532 eV is for a guide to the eye. G


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The Journal of Physical Chemistry C Table 1. Binding Energies for SiS ZnO as a Function of Growth Obtained from Fitsa cycles

Oc (eV)

Ob (eV)

1 1.5 2 2.5 3 3.5 4 6 8 12

532.98 534.05

531.65 531.82 531.96 532.02 531.87 532.19 532.25 531.95 531.82 532.1

SiS ZnO (this work) literature

Oa (eV)

intensity [Oc/Ob]

area [Oc/Ob]

0.33 0.10 0.00 0.00

0.48 0.07 0.00 0.00

530.49 530.82 530.68 530.54 530.36 530.47

intensity [Oa/Ob]

area [Oa/Ob]

0.00 0.00 0.29 0.76 1.03 1.09 1.16 1.93

0.00 0.00 0.16 0.52 0.71 0.86 0.98 1.80

Oc

Ob

Oa

533.5 ± 0.8 532.6 ± 0.6

532.0 ± 0.2 531.4 ± 0.3

530.6 ± 0.2 530.1 ± 0.2

a

All peaks calibrated to C1s at 284.6 eV and Oa, Ob, and Oc designations are according to ref 34. Literature values taken from refs 34, 40, 43, 44, and 51−56.

Information). This plot is once again in agreement with all of the PL, Raman, and XPS Zn data discussed above. There is clearly a transition from monomer Zn atom configurations (1 and 1.5 cycles), through a unique state with Zn dimer atom configurations (2 and 2.5 cycles), through a transition region that leads toward a bulk-like state after 6 cycles. The uniqueness of the 2- and 2.5-cycle SiS ZnO led to additional time-resolved PL characterization, Figures S10 and S11 (Supporting Information). The data shows a very fast, sub5 ps, absorption edge and a 9 ps decay for the 375 nm UV emission. This is quite fast and comparable with results shown for crystalline ZnO nanorods at 7 ps57 but faster than bulk ZnO at ∼30 ps.58,59

first confirmation of energy transfer between polymer and ZnO, where the polymer absorbs light at 241 nm and emits at 360 nm, which coincides with the ZnO UV emission peak. Our work shows that the ZnO dimers (2 and 2.5 SiS cycles) are unique configurations with high PL intensity, unique O1s oxidation states, and sub-10 ps absorption and decay, which are interesting properties for novel quantum material applications. We also have found that SiS ZnO is not a standard form of ZnO normally studied before. The fact that SiS ZnO can be defined in a lithographically patternable polymer, such as PMMA, raises possibilities of further exploitation of the SiS ZnO dimers as quantum materials that could be part of optical cavities or other photonic devices.

2. SUMMARY AND CONCLUSIONS We have investigated a variation of atomic layer deposition (ALD), called sequential infiltration synthesis (SiS), as an alternate method to incorporate ZnO inside poly(methyl methacrylate) (PMMA). Energy dispersive spectroscopy (EDS) results show that ZnO is synthesized up to 300 nm inside a PMMA film. A key feature of an ALD process is the ability to add an atomic layer at a time given that it is a selfterminating reaction after each cycle. This characteristic allows for a detailed study of the formation of ZnO in the polymer matrix after each atomic step is formed. Each growth step of ZnO in PMMA was characterized using ex-situ energy dispersive, photoluminescence, Raman, and X-ray photoemission spectroscopy. The results obtained show clear differences between mono, dimer, and trimer Zn atom configurations from the bulk. Mono Zn atoms (O−Zn and O−Zn−O) are formed with a single DEZ precursor pulse and one or two H2O pulses and exhibit pure UV emission with little evidence of deep level oxygen vacancy states (VO). Dimer Zn atoms (O−Zn−O−Zn and O−Zn−O−Zn−O) are formed with two pulses of DEZ and two or three pulses of H2O. The early ZnO configurations (mono and dimer ZnO atoms) are found to be isolated entities not forming a continuous film, as the Raman spectroscopy and XPS data shows no evidence of phonons or a ZnO wurtzite structure respectively. The dimer configuration shows strong UV and visible PL emission from VO states that is over 20 times greater than that from the mono Zn atom configuration. After 3 precursor cycles, first evidence of wurtzite formation inside the polymer matrix is observed with Raman spectroscopy and XPS. We also report a

3. METHODS Samples were prepared by spin coating substrates with 950 K PMMA dissolved in anisole as a casting solvent at 2000 rpm, and baked at 180 °C for 3 min. Typical thickness used was 250 nm. A Gemstar-8 thermal ALD tool from Arradiance was used for the SiS process. The Arradiance tool allows for exhaust valve shutoff during precursor exposure to the sample, which is critical for the infiltration process to proceed, as it allows the precursors time to diffuse into the polymer matrix. The SiS process is carried out at 95 °C which is below the glass transition temperature (Tg) of many polymers such as PMMA (Tg PMMA = 105 °C). The precursors used were water (H2O), tetramethylaluminum (TMA), and diethylzinc (DEZ), which were purchased from Strem Chemical. Given the known use of having Al2O3 as an adhesion layer for subsequent oxide layers,16 2 cycles of [H2O:TMA] were introduced for Al2O3 SiS processing prior to following cycles of [H2O:DEZ] for ZnO SiS processing. A half cycle consists of precursor injection in a series of short pulses of the precursor, followed by 2−4 min of precursor residency in the chamber, and a nitrogen flush for 40 s followed by a 3 s pump chamber down prior to the next injection. For the Al2O3 adhesion layer, 8 short pulses of each precursor (H2O and TMA) were injected per half cycle. For the ZnO, 8 short pulses of H2O and 11 pulses of DEZ were injected per half cycle. A Nanolog spectrofluorometer from Horiba Scientific was used for PL measurements. The tool consists of a broadband Hg source, with a monochromator, and a separate detection spectrometer. Emission data was acquired for multiple excitation wavelengths and plotted in a 3D format (emission H



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intensity as a function of excitation and emission wavelengths). The emission and excitation detectors were dark-currentcorrected, and then, the emission data was normalized to the intensity of the Hg lamp measured by the excitation detector. The excitation detector data is measured in μA. The emission detector data is measured in counts per second (cps). Therefore, the units of the PL maps are in cps/μA. The excitation light was incident normal to the sample surface, and the emission light was detected at a 22.5° angle with a 295 nm, or 305 nm, high pass filter in front. An in-house XPS/UPS tool was used. The XPS/UPS instrument is equipped with an X-ray gun (PHI, model 04500), VUV source (Focus Inc.), hemispherical energy analyzer (VSW, model HA-100), ion gun (PHI, model 04-303), electron gun (VG model LEG200), and LEED system (VG, model RVL900). A K-alpha Mg source was used. The power source was run at 15 kV, 300 W.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08007. Additional data on EDS cross sections for other polymers and a comparison of different process conditions where the purge time between precursor pulses was increased by a factor of 10; bandgap measurement data, additional photoluminescence data, a complete list of PL emission peaks found in the literature, and additional XPS experimental and reference data; and all of the fit parameters used in the discussion part of this paper (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1 630 252-6613. Fax: 1 630 252-5735. Web page: nano.anl.gov. ORCID

Leonidas E. Ocola: 0000-0003-4990-1064 Author Contributions

L.E.O. is the main author, oversaw the project, and contributed with overall work, photoluminescence, Raman, and XPS data. A.C. contributed with Raman spectroscopy and band gap data. D.J.D. contributed with photoluminescence and Raman work. R.D.S. contributed with time-resolved photoluminescence data. A.Y.-G. contributed with the EXAFS data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was also supported by the University of Chicago Materials Research Center (MRSEC) IRG3-Engineering Quantum Materials and Interactions Contract #2-60700-95. The authors acknowledge the support of Alex Zinovev for access and training on the XPS tool for this work. I


Article

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