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Highly transparent metal electrodes via direct printing processes
Accepted Manuscript Title: Highly transparent metal electrodes via direct printing processes Authors: Florian M. Wisser, Kai Eckhardt, Winfried Nickel, Winfried Bonullhlmann, Stefan Kaskel, Julia Grothe PII: DOI: Reference:
S0025-5408(17)32367-X https://doi.org/10.1016/j.materresbull.2017.10.021 MRB 9628
To appear in:
MRB
Received date: Revised date: Accepted date:
16-6-2017 21-8-2017 13-10-2017
Please cite this article as: Florian M.Wisser, Kai Eckhardt, Winfried Nickel, Winfried Box308;hlmann, Stefan Kaskel, Julia Grothe, Highly transparent metal electrodes via direct printing processes, Materials Research Bulletin https://doi.org/10.1016/j.materresbull.2017.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly transparent metal electrodes via direct printing processes Florian M. Wisser a,†, Kai Eckhardt a, Winfried Nickel a, Winfried Böhlmann b, Stefan Kaskel a, and Julia Grothe a,* a
Inorganic Chemistry I, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany, EMail: [email protected] b
Faculty of Physics and Earth Science, University of Leipzig, Linnéstraße 5, 04103 Leipzig, Germany †
Dr. Florian M. Wisser, present address: Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON - UMR 5256, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France.
Graphical Abstract
Highlights
Pt(NO3)2(ACN)2 as a new molecular platinum complex has been introduced
The complex shows excellent printability in nanoimprint lithography Pt line and grid pattern with linewidths as small as 40 nm could be printed The printed electrodes show resistance below 100 Ω and transmittance up to 90 %
Abstract Transparent platinum electrodes are prepared using nanoimprint lithography of a newly developed molecular platinum complex. Thermal decomposition at moderate temperature of this complex gives rise to elemental platinum line and grid pattern with linewidths down to 40 nm. These electrodes exhibit transmittances of 90 % and resistances below 100 Ω. Thus, for the first time, highly transparent electrodes are prepared by a direct soft lithographical printing process. Keywords: A. electrical materials; A.optical materials; A.thin films; B. optical properties; D. electrical properties 1. Introduction
1
Nanostructured metal electrodes have emerged as a new class of transparent conductive electrodes (TCE) for optoelectronic devices such as photovoltaic cells, light-emitting or organic light-emitting diodes [1,2]. Coating of silver or copper nanowire dispersions on substrates yields transparent nanowire meshes (T > 90 %) with sheet resistances down to 20 Ω [3,4]. The main drawbacks of metal nanowire networks are their susceptibility to corrosion and their low adhesion to the substrate [4]. Recently, soft lithographic methods emerged as promising alternative to selectively deposited metals as line or grid patterns for TCEs [5–8]. The major advantages of these techniques are the direct deposition, the periodicity and the high resolution of the patterns with linewidths even below 100 nm, resulting in transmittances of up to 84 % in the visible range [6,7,9]. For instance, an organometallic precursor or a metal nanoparticle ink can be structured using either micromolding in capillaries (MIMIC) or nanoimprint lithography (NIL) including solvent assisted nanoimprint lithography (SANIL). After decomposition of the organic moiety, pure metal structures are generated. The resulting patterns show good adhesion to the substrate and low sheet resistances [10]. Radha et al. used MIMIC of a “metal anion alkyl ammonium complex” for the preparation of platinum line patterns with a specific resistance of 2.8·10-6 Ω·m (bulk: 1.05·10-7 Ω·m) [9]. Recently, we reported on the use of various silver complexes as well as polymeric precursors containing either silver or platinum for soft lithographic applications. So far, only semitransparent electrodes have been achieved with these methods. The reason is that imprinting into a compact film of inks with high viscosity results in an incomplete displacement of the ink [11–13]. However, the performance of soft lithographic methods for the preparation of nano-structured metal electrodes was demonstrated. Compared to NIL, other soft lithographic techniques such as MIMIC are timeconsuming and not compatible to roll-to-roll processes. The potential of NIL in roll-to-roll processes has been demonstrated to prepare line patterns with feature sizes below 100 nm [14–16]. Moreover,
2
Wang et. al. have recently shown that NIL processes could easily be adapted to prepare homogeneous pattern in the sub-micrometer range on top of uneven substrates [17]. To achieve a high degree of transparency, we have focused our work on reducing the size of the metal features using NIL techniques. Here the primary aim is to enhance the transmittance while maintaining the electrical performance of the electrodes. To do so, we have developed a new molecular platinum precursor with a low viscosity. Platinum is favorable compared to cheaper silver or copper due to its significantly higher redox-potential and thus its higher stability against oxidation. Platinum allows for the direct preparation of carbon free structures, which is crucial for a good performance, but hard to achieve using silver or cooper in a direct sub-µm printing process [7,18]. In the following we demonstrate the excellent performance of this precursor in the generation of electrodes showing high transmittances up to 90 % and a remarkably low sheet resistance below 100 Ω. 2. Experimental 2.1 Materials Acetonitrile (HPLC grade) was purchased from VWR, platinum(II)chloride hydrate (99.9 %) and platinum(II)nitrate hydrate (57.81 % Pt) from ChemPur and boro-aluminosilicate glass (Corning 1737) from DELTA Technologies. Biaxially oriented polyethylene naphtalate foil (TEONEX Q65HA, 125 µm) was kindly donated from DuPont Teijin Films. 2.2 Synthetic procedures methods Bis(acetonitrile)dichloroplatinum(II) The synthesis was done following a published procedure [19]. Bis(acetonitrile)dinitratoplatinum(II)
3
In a 50 ml round bottom flask 315.8 mg (0.94 mmol) Pt(NO3)2 were dissolved in 20 ml acetonitrile and heated to reflux for 1 h. After cooling to room temperature, the solvent was removed under vacuum. The amorphous orange solid was dried under high vacuum. Yield: 317.4 mg (0.79 mmol, 84 %). 195
Pt MAS NMR (400 MHz, δ): -190 ppm;
13
C MAS NMR (400 MHz, δ): 178.0, 21.1 ppm;
IR (KBr): ν = 2996 (w), 2933 (w), 2341 (w; ν(CN)), 2312 (w; ν(CN)), 1566 (m), 1488 (s, broad, νas(NO3)), 1421, 1384, 1312, 1037, 940 (m), 820 (m), 704, 592 (m, broad), 308 cm−1 (s); Ink preparation and NIL 100 mg of bis(acetonitrile)dinitratoplatinum(II) were dissolved in either 320 µl DMF or 270 µl DMSO to give a brownish solution (10 wt.-% Pt). NIL was carried out using a µ-CP 3.0 from Gesim mbH. Detailed information on the stamp fabrication can be found in previous publications [20,21]. The stamp was pressed in a 3 µl droplet of the ink on top of the substrate and the ink was cured at 80 °C for 5-10 min (DMF). After removal of the stamp, the cured precursor was thermally reduced to elemental platinum under synthetic air at temperatures between 250 °C (PI substrates) and 600 °C (glass substrates) for 2 h (heating rate 1 K/min). 2.3 Instrumental analysis TG-DTA was carried out using a STA 409 (Netzsch) from RT to 400 °C with a heating rate of 1 K/min under synthetic air (5.0). Solid state MAS (magic angle spinning) NMR spectra were recorded using a Bruker Avance 400 (13C at 100.62 MHz, 195Pt at 86.02 MHz). They were referenced against adamantane (28.1 ppm and 37.2 ppm) and a 1.2 M H2PtCl6 in D2O (0 ppm) as external standard. 13C CP (cross polarization) MAS spectra were recorded at a rotation frequency of 10 kHz, 2048 scans were accumulated at 298 K
4
using a 𝜋/2 pulse of 7.5 µs. 195Pt MAS spectra were performed at a rotation frequency of 12 kHz, 4096 scans were accumulated at 298 K with a recycle delay 5 s and 𝜋/2 pulse of 7.2 µs. Atomic force microscope (AFM) measurements in the tapping mode were carried out using a Dimension 3100 (Veeco Nanoprobe tip RTESP7) connected to a Nanoscope IV SPM controller from Veeco Digital Instruments. Scanning electron microscopy (SEM) was performed using a ZEISS DSM982 Gemini. Transmittance spectra were recorded using a Cary 4000 (Varian), reflectance spectra using a Cary 5000 (Varian) equipped with an external Diffuse Reflectance Accessory DRA 2500 and an Ulbrichtsphere. IR spectra were recorded on a Bruker Vertex 70 in transmittance mode either as KBr pellet (MIR, 32 scans, resolution: 2 cm-1) or as CsI pellet (FIR, 128 scans, resolution: 2 cm-1). The resistance over the entire platinum pattern was measured between two opposite sides of the imprinted area using a two point probe multimeter [11]. 3. Results and discussion Starting from the literature-known platinum complex bis(acetonitrile)dichloroplatinum(II) [19], a new platinum complex bis(acetonitrile)dinitratoplatinum(II) (Pt(NO3)2(ACN)2) was synthesized as an orange, amorphous powder (Scheme 1) [22]. Changing the counterion from chlorine to nitrate avoids a poisoning of the Pt surface with remaining chlorine atoms. Moreover, thermal decomposition of Pt(NO3)2(ACN)2 to elemental Pt(0) is completed below 220 °C and thus ~180 °C lower as compared to PtCl2(ACN)2 (Figure S3). A decomposition temperature below the glass transition temperature of high temperature-stable polymers (TG > 250 °C) is of vital importance for the preparation of TCE on polymeric substrates such as polyimides, cycloolefine-copolymers or poly(ethersulfone) as well as biaxially oriented polyethylene naphthalate (PEN) [23–25].
5
For the preparation of a printable ink, Pt(NO3)2(ACN)2 was dissolved in DMF or DMSO. For DMF as solvent, the temperature for complete decomposition of the complex to Pt(0) did not change, whereas for DMSO, a higher temperature (350 °C) was required, clearly pointing towards DMF as the better choice for the use in NIL (Figure S4). Metal loadings up to 20 % Pt in the final ink were achieved. A solution of the complex in DMF was used as ink in solvent assisted NIL resulting in different line and grid patterns. We note, that the ink was completely cured after 5 minutes. However, longer heating times were applied to enhance particle sintering and to reduce the number of grain boundaries. Line pattern with line widths of 200 nm (period: 1000 nm) and 0.61 µm (period: 2 µm) were initially prepared on glass substrates (Figure 1). For both structures, homogeneous patterns were achieved, with a resistance as low as 95 Ω over the imprinted area of 1 x 1 cm (Table 1). Taking into account the height (15 nm, determined by AFM) of the platinum lines and the filling factor, the specific resistance of the 2 µm period pattern was calculated to be 4.3·10-7 Ω·m [9]. Compared to platinum line patterns with comparable dimensions reported by Radha et al. and Greco et al., this is an enhancement of at least a factor of six in specific resistance [9,10]. This remarkable improvement is most likely caused by a higher degree of crystallinity confirmed by much sharper XRD signals (Figure S5). Even for the smaller platinum line pattern, the transmittance is 79 % as compared to the transmission of the substrate (Table 1, Figure S6). Most of the decrease in transmittance is caused by additional reflectance at the structured surface (up to 20 %, Figure S7). Compared to the best NILprinted TCE from literature we reported an increase in transmittance of 10 %. In addition, high quality line patterns with a width of 0.38 µm (period: 1.91 µm) were successfully generated on transparent PEN substrates (Figure 1), achieving a transmittance of about 80 % in comparison to the substrate (Figure S8). This is a significant enhancement in transmittance compared to other metal patterns prepared by NIL, which have so far reached merely 60 % [11].
6
However, a further increase to about 90 % is required and the resistivity over the imprinted area (10 kΩ) is still higher than the target value of 100 Ω. The specific resistance could be calculated to be 9.6·10-5 Ω·m, a value comparable to that reported by Greco et al. for structures with similar dimensions [10]. The higher sheet resistance as compared to the patterns on glass substrates is most likely caused by the lower heating temperature (250 °C vs. 600 °C) resulting in less sintering of the platinum particles [10,11]. The slightly smaller period as compared to the 2 µm period pattern on glass substrate is most likely caused by shrinkage of the polymeric substrate. To boost the Pt-NIL performance even further, we decided to prepare grid pattern with different line widths and square based windows: a pattern with 40 nm line width (window width: 1100 nm, called small grid in the following), and a pattern with 0.67 µm line width (window width: 2.2 µm, period: 2.87 µm, called large grid) on glass substrates. Again, homogeneous imprinted pattern consisting of phase pure Pt (Figure S5) over the printing area of 1 x 1 cm were obtained (Figure 2, Figure S9). We like to point out, that for the first time, pattern with line widths below 100 nm have been printed successfully in a one-step direct printing process. The resistance over the imprinted area is as low as 100 ± 5 Ω for the small and 120 ± 20 Ω for the large grid (Table 1). To calculate the specific resistance of the grid pattern, we used the equation 𝑹 =
𝝆·𝑳 𝒉·𝝎
as introduced
by van de Groep et al. [26]. Here, R describes the sheet resistance, ρ the specific resistance, L the window width, h the height and ω the linewidth. A specific resistance as low as 2.2·10-7 Ω·m was obtained for the small grid grid (5.5·10-7 Ω·m for the large grid), already close to the resistance of the bulk material (1.05 10-7 Ω·m) [27]. It has to be noted, that the specific resistance of the material used has a large impact on the sheet resistance of the printed electrode once the geometry of the pattern has been optimized for high transmittance and should not been changed (vide infra). Thus, the different materials as well as geometries of TCEs reported in literature make it unfeasible to compare them in terms of a single value, e.g. figure of merit [8].
7
The transmittance increased with decreasing line width from approximately 68 % for the large to 90 % for the small grid (Table 1, Figure 3, Figure S10), compared to the transmission of the substrate. Here we note that the transmission is constant over a wide wavelength regime (400 to 900 nm). To the best of our knowledge this is the first time that structures with transmittances of 90 %, which fulfill the requirements for TCEs, have been obtained using a simple NIL process. For a 51.3 nm silver grid (window width: 1000 nm), a transmittance of approximately 85 % has been calculated [28], which is in good agreement with the obtained values. Thus a crucial achievement with the new precursor is the almost complete displacement of ink in the window area during the printing process resulting in highly transparent structured electrodes (Figure 2). 4. Conclusion In this contribution, we introduced Pt(NO3)2(ACN)2 as a new precursor showing excellent printability in NIL and allowing to prepare Pt patterns with linewidths as small as 40 nm. The subtle interplay of sophisticated precursor development and processing techniques is a crucial requirement for the costcompetitive preparation of transparent and highly conductive electrodes based on metals. The newly developed electrodes show specific resistances very close to that of the bulk material (as low as 2.2·10-7 Ω·m), which has not been achieved before using soft lithographic techniques. Thus, for the first time a resistance below 100 Ω combined with transmittance up to 90 % over the imprinted area could be achieved using NIL. In addition, the procedure described herein is promising for adaptation to other noble metal nitrile-complexes such as succinonitrilo-disilver(I) nitrate or bis(acetonitrile)dinitrato-copper(II) [29,30]. However, the main problems of oxidation during decomposition and remaining carbon within the structures will have to be overcome [7,18]. Acknowledgment The authors are grateful to I. Kunert for thermal analyses and A. Wolf for reflectance measurements. Supporting information
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Supporting Information associated with this article can be found online. It includes additional selectivity data. Furthermore an estimation and comparison of cost relative to ITO and 195Pt as well as 13C MAS NMR spectra are given.
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Figure 1. SEM images of line patterns with a line width of (a, b) 0.61 µm (period: 2 µm) and (c, d) 200 nm (period: 1000 nm) on corning glass as well as (e, f) 0.38 µm (period: 1.9 µm) on PEN.
Figure 2. SEM images of the large (a, b) and small grid (c, d) on corning glass.
Figure 3. Transmittance (black) and reflectance (gray) spectra of the small and the large grid.
13
Scheme 1. Reactionscheme for the synthesis of bis(acetonitrile)dinitratoplatinum(II).
14
Table 1. Comparison of transmittance (average between 400 and 800 nm) and sheet resistance of the different TCE on glass substrate. The transmittance values were corrected with respect to the substrate. Pt-line pattern
Pt-grid pattern
ITO 200 nm
610 nm
small
large
R/Ω
26
> 1000
95 ± 2
100 ± 5
120 ± 20
T/%
92
79
66
89
68
15
S0025-5408(17)32367-X https://doi.org/10.1016/j.materresbull.2017.10.021 MRB 9628
To appear in:
MRB
Received date: Revised date: Accepted date:
16-6-2017 21-8-2017 13-10-2017
Please cite this article as: Florian M.Wisser, Kai Eckhardt, Winfried Nickel, Winfried Box308;hlmann, Stefan Kaskel, Julia Grothe, Highly transparent metal electrodes via direct printing processes, Materials Research Bulletin https://doi.org/10.1016/j.materresbull.2017.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly transparent metal electrodes via direct printing processes Florian M. Wisser a,†, Kai Eckhardt a, Winfried Nickel a, Winfried Böhlmann b, Stefan Kaskel a, and Julia Grothe a,* a
Inorganic Chemistry I, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany, EMail: [email protected] b
Faculty of Physics and Earth Science, University of Leipzig, Linnéstraße 5, 04103 Leipzig, Germany †
Dr. Florian M. Wisser, present address: Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON - UMR 5256, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France.
Graphical Abstract
Highlights
Pt(NO3)2(ACN)2 as a new molecular platinum complex has been introduced
The complex shows excellent printability in nanoimprint lithography Pt line and grid pattern with linewidths as small as 40 nm could be printed The printed electrodes show resistance below 100 Ω and transmittance up to 90 %
Abstract Transparent platinum electrodes are prepared using nanoimprint lithography of a newly developed molecular platinum complex. Thermal decomposition at moderate temperature of this complex gives rise to elemental platinum line and grid pattern with linewidths down to 40 nm. These electrodes exhibit transmittances of 90 % and resistances below 100 Ω. Thus, for the first time, highly transparent electrodes are prepared by a direct soft lithographical printing process. Keywords: A. electrical materials; A.optical materials; A.thin films; B. optical properties; D. electrical properties 1. Introduction
1
Nanostructured metal electrodes have emerged as a new class of transparent conductive electrodes (TCE) for optoelectronic devices such as photovoltaic cells, light-emitting or organic light-emitting diodes [1,2]. Coating of silver or copper nanowire dispersions on substrates yields transparent nanowire meshes (T > 90 %) with sheet resistances down to 20 Ω [3,4]. The main drawbacks of metal nanowire networks are their susceptibility to corrosion and their low adhesion to the substrate [4]. Recently, soft lithographic methods emerged as promising alternative to selectively deposited metals as line or grid patterns for TCEs [5–8]. The major advantages of these techniques are the direct deposition, the periodicity and the high resolution of the patterns with linewidths even below 100 nm, resulting in transmittances of up to 84 % in the visible range [6,7,9]. For instance, an organometallic precursor or a metal nanoparticle ink can be structured using either micromolding in capillaries (MIMIC) or nanoimprint lithography (NIL) including solvent assisted nanoimprint lithography (SANIL). After decomposition of the organic moiety, pure metal structures are generated. The resulting patterns show good adhesion to the substrate and low sheet resistances [10]. Radha et al. used MIMIC of a “metal anion alkyl ammonium complex” for the preparation of platinum line patterns with a specific resistance of 2.8·10-6 Ω·m (bulk: 1.05·10-7 Ω·m) [9]. Recently, we reported on the use of various silver complexes as well as polymeric precursors containing either silver or platinum for soft lithographic applications. So far, only semitransparent electrodes have been achieved with these methods. The reason is that imprinting into a compact film of inks with high viscosity results in an incomplete displacement of the ink [11–13]. However, the performance of soft lithographic methods for the preparation of nano-structured metal electrodes was demonstrated. Compared to NIL, other soft lithographic techniques such as MIMIC are timeconsuming and not compatible to roll-to-roll processes. The potential of NIL in roll-to-roll processes has been demonstrated to prepare line patterns with feature sizes below 100 nm [14–16]. Moreover,
2
Wang et. al. have recently shown that NIL processes could easily be adapted to prepare homogeneous pattern in the sub-micrometer range on top of uneven substrates [17]. To achieve a high degree of transparency, we have focused our work on reducing the size of the metal features using NIL techniques. Here the primary aim is to enhance the transmittance while maintaining the electrical performance of the electrodes. To do so, we have developed a new molecular platinum precursor with a low viscosity. Platinum is favorable compared to cheaper silver or copper due to its significantly higher redox-potential and thus its higher stability against oxidation. Platinum allows for the direct preparation of carbon free structures, which is crucial for a good performance, but hard to achieve using silver or cooper in a direct sub-µm printing process [7,18]. In the following we demonstrate the excellent performance of this precursor in the generation of electrodes showing high transmittances up to 90 % and a remarkably low sheet resistance below 100 Ω. 2. Experimental 2.1 Materials Acetonitrile (HPLC grade) was purchased from VWR, platinum(II)chloride hydrate (99.9 %) and platinum(II)nitrate hydrate (57.81 % Pt) from ChemPur and boro-aluminosilicate glass (Corning 1737) from DELTA Technologies. Biaxially oriented polyethylene naphtalate foil (TEONEX Q65HA, 125 µm) was kindly donated from DuPont Teijin Films. 2.2 Synthetic procedures methods Bis(acetonitrile)dichloroplatinum(II) The synthesis was done following a published procedure [19]. Bis(acetonitrile)dinitratoplatinum(II)
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In a 50 ml round bottom flask 315.8 mg (0.94 mmol) Pt(NO3)2 were dissolved in 20 ml acetonitrile and heated to reflux for 1 h. After cooling to room temperature, the solvent was removed under vacuum. The amorphous orange solid was dried under high vacuum. Yield: 317.4 mg (0.79 mmol, 84 %). 195
Pt MAS NMR (400 MHz, δ): -190 ppm;
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C MAS NMR (400 MHz, δ): 178.0, 21.1 ppm;
IR (KBr): ν = 2996 (w), 2933 (w), 2341 (w; ν(CN)), 2312 (w; ν(CN)), 1566 (m), 1488 (s, broad, νas(NO3)), 1421, 1384, 1312, 1037, 940 (m), 820 (m), 704, 592 (m, broad), 308 cm−1 (s); Ink preparation and NIL 100 mg of bis(acetonitrile)dinitratoplatinum(II) were dissolved in either 320 µl DMF or 270 µl DMSO to give a brownish solution (10 wt.-% Pt). NIL was carried out using a µ-CP 3.0 from Gesim mbH. Detailed information on the stamp fabrication can be found in previous publications [20,21]. The stamp was pressed in a 3 µl droplet of the ink on top of the substrate and the ink was cured at 80 °C for 5-10 min (DMF). After removal of the stamp, the cured precursor was thermally reduced to elemental platinum under synthetic air at temperatures between 250 °C (PI substrates) and 600 °C (glass substrates) for 2 h (heating rate 1 K/min). 2.3 Instrumental analysis TG-DTA was carried out using a STA 409 (Netzsch) from RT to 400 °C with a heating rate of 1 K/min under synthetic air (5.0). Solid state MAS (magic angle spinning) NMR spectra were recorded using a Bruker Avance 400 (13C at 100.62 MHz, 195Pt at 86.02 MHz). They were referenced against adamantane (28.1 ppm and 37.2 ppm) and a 1.2 M H2PtCl6 in D2O (0 ppm) as external standard. 13C CP (cross polarization) MAS spectra were recorded at a rotation frequency of 10 kHz, 2048 scans were accumulated at 298 K
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using a 𝜋/2 pulse of 7.5 µs. 195Pt MAS spectra were performed at a rotation frequency of 12 kHz, 4096 scans were accumulated at 298 K with a recycle delay 5 s and 𝜋/2 pulse of 7.2 µs. Atomic force microscope (AFM) measurements in the tapping mode were carried out using a Dimension 3100 (Veeco Nanoprobe tip RTESP7) connected to a Nanoscope IV SPM controller from Veeco Digital Instruments. Scanning electron microscopy (SEM) was performed using a ZEISS DSM982 Gemini. Transmittance spectra were recorded using a Cary 4000 (Varian), reflectance spectra using a Cary 5000 (Varian) equipped with an external Diffuse Reflectance Accessory DRA 2500 and an Ulbrichtsphere. IR spectra were recorded on a Bruker Vertex 70 in transmittance mode either as KBr pellet (MIR, 32 scans, resolution: 2 cm-1) or as CsI pellet (FIR, 128 scans, resolution: 2 cm-1). The resistance over the entire platinum pattern was measured between two opposite sides of the imprinted area using a two point probe multimeter [11]. 3. Results and discussion Starting from the literature-known platinum complex bis(acetonitrile)dichloroplatinum(II) [19], a new platinum complex bis(acetonitrile)dinitratoplatinum(II) (Pt(NO3)2(ACN)2) was synthesized as an orange, amorphous powder (Scheme 1) [22]. Changing the counterion from chlorine to nitrate avoids a poisoning of the Pt surface with remaining chlorine atoms. Moreover, thermal decomposition of Pt(NO3)2(ACN)2 to elemental Pt(0) is completed below 220 °C and thus ~180 °C lower as compared to PtCl2(ACN)2 (Figure S3). A decomposition temperature below the glass transition temperature of high temperature-stable polymers (TG > 250 °C) is of vital importance for the preparation of TCE on polymeric substrates such as polyimides, cycloolefine-copolymers or poly(ethersulfone) as well as biaxially oriented polyethylene naphthalate (PEN) [23–25].
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For the preparation of a printable ink, Pt(NO3)2(ACN)2 was dissolved in DMF or DMSO. For DMF as solvent, the temperature for complete decomposition of the complex to Pt(0) did not change, whereas for DMSO, a higher temperature (350 °C) was required, clearly pointing towards DMF as the better choice for the use in NIL (Figure S4). Metal loadings up to 20 % Pt in the final ink were achieved. A solution of the complex in DMF was used as ink in solvent assisted NIL resulting in different line and grid patterns. We note, that the ink was completely cured after 5 minutes. However, longer heating times were applied to enhance particle sintering and to reduce the number of grain boundaries. Line pattern with line widths of 200 nm (period: 1000 nm) and 0.61 µm (period: 2 µm) were initially prepared on glass substrates (Figure 1). For both structures, homogeneous patterns were achieved, with a resistance as low as 95 Ω over the imprinted area of 1 x 1 cm (Table 1). Taking into account the height (15 nm, determined by AFM) of the platinum lines and the filling factor, the specific resistance of the 2 µm period pattern was calculated to be 4.3·10-7 Ω·m [9]. Compared to platinum line patterns with comparable dimensions reported by Radha et al. and Greco et al., this is an enhancement of at least a factor of six in specific resistance [9,10]. This remarkable improvement is most likely caused by a higher degree of crystallinity confirmed by much sharper XRD signals (Figure S5). Even for the smaller platinum line pattern, the transmittance is 79 % as compared to the transmission of the substrate (Table 1, Figure S6). Most of the decrease in transmittance is caused by additional reflectance at the structured surface (up to 20 %, Figure S7). Compared to the best NILprinted TCE from literature we reported an increase in transmittance of 10 %. In addition, high quality line patterns with a width of 0.38 µm (period: 1.91 µm) were successfully generated on transparent PEN substrates (Figure 1), achieving a transmittance of about 80 % in comparison to the substrate (Figure S8). This is a significant enhancement in transmittance compared to other metal patterns prepared by NIL, which have so far reached merely 60 % [11].
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However, a further increase to about 90 % is required and the resistivity over the imprinted area (10 kΩ) is still higher than the target value of 100 Ω. The specific resistance could be calculated to be 9.6·10-5 Ω·m, a value comparable to that reported by Greco et al. for structures with similar dimensions [10]. The higher sheet resistance as compared to the patterns on glass substrates is most likely caused by the lower heating temperature (250 °C vs. 600 °C) resulting in less sintering of the platinum particles [10,11]. The slightly smaller period as compared to the 2 µm period pattern on glass substrate is most likely caused by shrinkage of the polymeric substrate. To boost the Pt-NIL performance even further, we decided to prepare grid pattern with different line widths and square based windows: a pattern with 40 nm line width (window width: 1100 nm, called small grid in the following), and a pattern with 0.67 µm line width (window width: 2.2 µm, period: 2.87 µm, called large grid) on glass substrates. Again, homogeneous imprinted pattern consisting of phase pure Pt (Figure S5) over the printing area of 1 x 1 cm were obtained (Figure 2, Figure S9). We like to point out, that for the first time, pattern with line widths below 100 nm have been printed successfully in a one-step direct printing process. The resistance over the imprinted area is as low as 100 ± 5 Ω for the small and 120 ± 20 Ω for the large grid (Table 1). To calculate the specific resistance of the grid pattern, we used the equation 𝑹 =
𝝆·𝑳 𝒉·𝝎
as introduced
by van de Groep et al. [26]. Here, R describes the sheet resistance, ρ the specific resistance, L the window width, h the height and ω the linewidth. A specific resistance as low as 2.2·10-7 Ω·m was obtained for the small grid grid (5.5·10-7 Ω·m for the large grid), already close to the resistance of the bulk material (1.05 10-7 Ω·m) [27]. It has to be noted, that the specific resistance of the material used has a large impact on the sheet resistance of the printed electrode once the geometry of the pattern has been optimized for high transmittance and should not been changed (vide infra). Thus, the different materials as well as geometries of TCEs reported in literature make it unfeasible to compare them in terms of a single value, e.g. figure of merit [8].
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The transmittance increased with decreasing line width from approximately 68 % for the large to 90 % for the small grid (Table 1, Figure 3, Figure S10), compared to the transmission of the substrate. Here we note that the transmission is constant over a wide wavelength regime (400 to 900 nm). To the best of our knowledge this is the first time that structures with transmittances of 90 %, which fulfill the requirements for TCEs, have been obtained using a simple NIL process. For a 51.3 nm silver grid (window width: 1000 nm), a transmittance of approximately 85 % has been calculated [28], which is in good agreement with the obtained values. Thus a crucial achievement with the new precursor is the almost complete displacement of ink in the window area during the printing process resulting in highly transparent structured electrodes (Figure 2). 4. Conclusion In this contribution, we introduced Pt(NO3)2(ACN)2 as a new precursor showing excellent printability in NIL and allowing to prepare Pt patterns with linewidths as small as 40 nm. The subtle interplay of sophisticated precursor development and processing techniques is a crucial requirement for the costcompetitive preparation of transparent and highly conductive electrodes based on metals. The newly developed electrodes show specific resistances very close to that of the bulk material (as low as 2.2·10-7 Ω·m), which has not been achieved before using soft lithographic techniques. Thus, for the first time a resistance below 100 Ω combined with transmittance up to 90 % over the imprinted area could be achieved using NIL. In addition, the procedure described herein is promising for adaptation to other noble metal nitrile-complexes such as succinonitrilo-disilver(I) nitrate or bis(acetonitrile)dinitrato-copper(II) [29,30]. However, the main problems of oxidation during decomposition and remaining carbon within the structures will have to be overcome [7,18]. Acknowledgment The authors are grateful to I. Kunert for thermal analyses and A. Wolf for reflectance measurements. Supporting information
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Supporting Information associated with this article can be found online. It includes additional selectivity data. Furthermore an estimation and comparison of cost relative to ITO and 195Pt as well as 13C MAS NMR spectra are given.
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Figure 1. SEM images of line patterns with a line width of (a, b) 0.61 µm (period: 2 µm) and (c, d) 200 nm (period: 1000 nm) on corning glass as well as (e, f) 0.38 µm (period: 1.9 µm) on PEN.
Figure 2. SEM images of the large (a, b) and small grid (c, d) on corning glass.
Figure 3. Transmittance (black) and reflectance (gray) spectra of the small and the large grid.
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Scheme 1. Reactionscheme for the synthesis of bis(acetonitrile)dinitratoplatinum(II).
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Table 1. Comparison of transmittance (average between 400 and 800 nm) and sheet resistance of the different TCE on glass substrate. The transmittance values were corrected with respect to the substrate. Pt-line pattern
Pt-grid pattern
ITO 200 nm
610 nm
small
large
R/Ω
26
> 1000
95 ± 2
100 ± 5
120 ± 20
T/%
92
79
66
89
68
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