Photolithography-compatible conformal electrodes for high-performance bottom-contact organic single-crystal transistors

Xiaoli Zhao , Xueyan Ding , Qingxin Tang *, Yanhong Tong and Yichun Liu
Key Laboratory of UV Light Emitting Materials and Technology under Ministry of Education, Northeast Normal University, Changchun 130024, P. R. China. E-mail: tangqx@nenu.edu.cn; Fax: +86-431-85099873; Tel: +86-431-85099873

Received 21st September 2017 , Accepted 12th November 2017

First published on 15th November 2017


High-performance rubrene single-crystal field-effect transistors (SCFETs) with bottom-gate bottom-contact configuration were successfully fabricated on both planar and curved surfaces based on a photolithography-compatible conformal electrode. This electrode not only provides versatile precise patterns for device design, but also eliminates the device differences by the fabrication of multiple devices based on one single crystal, which is very favorable for studies of the intrinsic properties and integration of organic devices. The resulting rubrene SCFETs show excellent electrical properties with good device uniformity, zero hysteresis, a device yield as high as 92%, and a field-effect mobility of over 20 cm2 V−1 s−1 on different surfaces including a banknote, a pencil, and a 0.7 cm glass sphere. The high electrical performance in our bottom-contact devices can be attributed to the nondestructive interface contact and eliminated electrode steps. Such a soft coplanar electrode provides a preferred configuration for bottom-contact organic field-effect transistors (OFETs), facilitating the studies on the fundamental properties of organic transistors, and showing strong potential for the development of large-scale commercial organic transistor fabrication.


Introduction

Organic semiconductors are considered to be the most promising channel materials for next-generation wearable and portable electronics because of their low Young's modulus, low cost, long-term biocompatibility, and simple packaging.1–5 Compared with their thin films, organic crystals possess some unique advantages, such as long-range order, the absence of grain boundaries, low-density defects, and a well-defined molecular packing structure, which enable organic single-crystal field-effect transistors (SCFETs) to not only acquire the best field-effect performance with the highest charge mobility, but also act as a versatile tool for revealing the intrinsic charge transport and the structure–property relationships of organic materials.6

To date, it is still a huge challenge to integrate bottom-contact configuration and high performance in organic field-effect transistors (OFETs). It is well known that the bottom-contact OFETs are favorable for the integration of organic devices and circuits compared with top-contact transistors. The small-size source/drain electrodes and wiring can be formed in bottom-contact devices by photolithography, whereas there is a serious limit to scale down the channel length and width in the top-contact configuration where the source/drain electrodes generally are fabricated by metal shadow masks. At the same time, the bottom-contact configuration is favorable for the study of the intrinsic properties of organic materials. A precise and complicated electrode pattern can be easily created so that multiple organic SCFETs with changed target parameters can be fabricated based on one organic crystal, which can effectively eliminate device differences caused by crystal differences like size and morphology differences. However, the bottom-contact OFETs generally show far lower mobility than the top-contact OFETs.7–11 The dilemma between the bottom-contact configuration and the device performance inevitably affects the reliability and comparability of the measured data for studies of intrinsic properties, and at the same time also brings a difficulty for future industrial applications of OFETs. Therefore, it is urgent to develop a new method to fabricate high-performance bottom-contact OFETs.

Here, we applied a photolithography-compatible electrode to successfully realize high-performance bottom-contact organic SCFETs. These transistors were constructed by laminating the organic crystal on the soft electrodes with bottom-gate bottom-contact coplanar configuration. This method not only can produce multiple devices with high precision for the elimination of device differences, but also can create nondestructive, soft, and non-step electrode contacts for high device performance. The resulting rubrene SCFETs show excellent electrical properties with good device uniformity, zero hysteresis, a device yield as high as 92%, and a field-effect mobility up to 24.5 cm2 V−1 s−1. The obtained mobility is among the highest values reported for rubrene SCFETs, and even higher than those of the reported top-contact SCFETs. It is worth mentioning that our organic SCFETs show outstanding conformability to static curved objects and at the same time maintain high performance. Altogether these results show that our photolithography-compatible coplanar electrode provides an effective way to fabricate a high-performance bottom-contact device for fundamental studies and practical applications.

Experimental section

Preparation of conformal electrodes

Gate electrodes and source/drain electrodes were separately prepared with an embedded structure, and then were laminated together. Peeling and adhesion between the layers were controlled by surface modification and treatment. Firstly, a 25 nm-thick Au gate electrode and a source/drain electrode pattern were separately fabricated on OTS (Acros, 95%) modified Si substrates via photolithography. Secondly, prior to the removal of the photoresist, the samples were exposed to 3-mercaptopropyltrimethoxysilane (MPT) (Sigma Aldrich, 95%) for intimate contact between Au and PDMS. Thirdly, polydimethylsiloxane (PDMS) (Dow Corning, Sylgard 184) solutions were respectively spin coated on the gate electrode and source/drain electrodes. The thickness of the PDMS dielectric is ∼5 μm. Dielectric capacitance was measured using a charge integration technique and was calculated by using the measured thickness and the known dielectric constant of PDMS at 2.73. The calculated values were at ∼0.4861 ± 0.03632 nF cm−2. Fourthly, the gate electrode embedded in PDMS was carefully peeled from the OTS/Si substrate, and then was laminated on the source/drain electrodes embedded in PDMS by plasma oxidation. We applied alignment patterns to realize the accurate lamination of the large-area conformal electrode. Based on alignment patterns, the electrode can be accurately aligned by an alignment tool to adjust the relative position of the gate electrode and the source/drain electrodes. The whole alignment process was performed under an optical microscope. Finally, the embedded laminated electrode was obtained by peeling off the whole structure (gate electrode embedded in PDMS and source/drain electrodes embedded in PDMS) from the OTS/Si substrate. The electrode was flipped over and its peeled surface was used to contact with the semiconductor for device fabrication.

Device fabrication and measurements

Based on the conformal electrode, rubrene SCFETs with bottom-gate bottom-contact configurations were successfully prepared. Rubrene single-crystal nanobelts were fabricated by a physical vapor transport method.12 Rubrene single crystals were grown by a physical vapor transport process in a horizontal tube furnace. High-purity rubrene (99.99%) was purchased from Sigma Aldrich. At first, rubrene powders were purified by sublimation in a high vacuum system. N-type (100) oriented Si wafers were used as the substrates in the low-temperature zone. A quartz boat with the purified rubrene powder was placed at the high-temperature zone and vaporized at 300 °C for 60 min. Rubrene single crystals were grown in a flow of high-purity nitrogen (99.999%) at ambient pressure. The flow rate of nitrogen was 150 mL min−1. For the obtained rubrene crystals, the length, width, and thickness ranges from several micrometers to several millimeters, several to hundreds of micrometers, and dozens to hundreds of nanometers, respectively. The distribution of rubrene crystals is mainly several hundred micrometers in length, dozens of micrometers in width, and hundreds of nanometers in thickness. In order to integrate multiple high-performance devices into a single crystal, we generally selected a crystal length over 100 μm, the width less than 10 μm, and a thickness over 100 nm. The conformal transistors were achieved by putting the rubrene nanobelt onto the conformal electrode with the tip of the mechanical probe. The electrical characteristics of the OFET devices were recorded using a Keithley 4200 SCS on a Cascade M150 probe station in a clean and shielded box at room temperature in air. The 3D optical images of the electrodes were obtained using a 3D digital microscope (Keyence, VHX-5000). AFM measurements were carried out on a Dimension Icon instrument using a NanoScopeV9 controller (Bruker, Inc.). All experiments were performed in compliance with the relevant laws and institutional guidelines, and Northeast Normal University confirmed that formal approval was not necessary for this study; the only participant in this study was one of the co-authors.

Results and discussion

Design and fabrication of the conformal electrode

In our experiments, we designed a photolithography-compatible conformal electrode for organic SCFETs. The conformal electrode was achieved by separately fabricating a soft gate electrode and source/drain electrodes by a photolithography technique, and laminating them together to form a monolithic electrode as shown in Fig. 1a (see the Experimental section for details). The SEM image of Fig. 1a shows a real conformal electrode and clearly demonstrates the double-layer electrode structure for transistor fabrication. As shown in the schematic diagram of Fig. 1a, photolithographic Au patterns are used as source/drain/gate electrodes, respectively. PDMS serves as the dielectric layer and the supporting layer when combined with source/drain and gate electrodes, respectively. The 3D optical microscopy image of Fig. 1b shows a typical conformal electrode with a complicated photolithographic pattern. The reflection (top right) and transmission (bottom right) optical images clearly show the electrode array without wrinkles and cracks on the electrode surface, which ensures the device yield of fabricating organic SCFETs. In the experimental process, no solvent or water was introduced in the peeling process, and no additional adhesive or glue was applied to connect the gate electrode and source/drain electrodes. The peeling and adhesion between layers were controlled by surface modification and treatment (Fig. S1, ESI). Such an all-dry manufacturing process not only avoids the use of toxic or environmentally harmful solvents, but also produces a cleaner contact interface for further device fabrication.
image file: c7tc04313a-f1.tif
Fig. 1 Embedded laminated electrodes for conformal electronics. (a) Schematic images of electrodes composed of photolithographic source/drain electrodes in PDMS dielectric and the photolithographic gate electrode in PDMS. The right SEM image shows a real electrode. (b) 3D optical microscopy image (left), and reflection (right top) and transmission (right bottom) optical images of the electrode arrays.

Different from the previously reported electrode fabricated technology of OFETs,13,14 our electrode of a thin size can spontaneously conform to an object with an arbitrary shape by van der Waals forces, which ensures the intimate contact between the electrode and the crystal for subsequent transistor preparation. To examine the conformal ability of our electrode, the electrode is adhered onto the movement of a human elbow (Fig. 2a). It can be gently attached to the skin or wrapped around limbs and does not make the wearer perceive any discomfort. As shown in Movie S1 (ESI), the electrode can be continuously bent so that it conforms to the movements of the human body, which has uneven surfaces with a large range of motion. No significant loss in conductivity is observed before and after the elbow movements as shown in Fig. S2 (ESI).


image file: c7tc04313a-f2.tif
Fig. 2 Characteristics of the photolithographic conformal electrode for OFETs. (a) Photographs showing that the electrode can well conform to the movement of a human elbow. (b) Optical microscopy images of three kinds of typical electrodes with different photolithographic patterns. Each of the electrode patterns is composed of separate gate and source/drain electrodes.

So far, most reported organic transistors generally adopt vacuum deposition with a shadow mask to prepare electrodes on the semiconductor.15–18 This electrode technology limits the miniaturization and integration of the devices and circuits.19 In contrast, photolithography can produce complex and precise patterns, favoring the fabrication of high-integration and multifunctional devices and circuits. However, the conventional photolithography technique requires the fabrication of electrodes on the top of the semiconductor,20–24 which inevitability introduces solvent pollution or damage on the organic semiconductor, resulting in performance deterioration and even performance destruction. Here, we develop a new-type electrode that is compatible with a photolithography technique and at the same time can be free from damage of the organic semiconductor. Fig. 2b shows three typical electrodes with different complicated photolithographic patterns. For each electrode pattern, the gate electrode (left top) and source/drain electrodes (right top) were laminated together to form the full-photolithographic monolithic electrode (bottom) for transistor fabrication. Our electrode technology separates the semiconductor fabrication from electrode preparation, and hence effectively avoids various types of damage and other unwanted changes to the fragile organic semiconductors in the process of device preparation making it possible to obtain large-area high-integration devices and circuits.

Organic SCFETs based on the conformal electrode

Based on our designed electrode pattern, organic SCFETs with bottom-gate top-contact configuration can be easily obtained by laminating the flexible rubrene single-crystal nanobelts onto the prepared conformal electrode. Fig. 3a clearly shows the schematic and the corresponding cross-sectional diagram of the device. The optical image of Fig. 3b shows the real rubrene SCFET arrays and the magnified view. In our experiments, rubrene is selected to be expended as the channel material, because rubrene is one of the most promising organic materials in desirable electrical performance. As shown in Fig. 3c, the bendable and flexible morphology of rubrene nanobelts shows that they can be used to fabricate OFETs on curved and flexible substrates to meet the requirements of conformal and flexible electronics. The length of rubrene nanobelts ranges from several micrometers to several millimeters, and the width ranges from several micrometers to hundreds of micrometers (Fig. 3c). As shown in Fig. 3d, the thickness is at a few hundreds of nanometers, and the average roughness on the surface of a typical rubrene nanobelt is only at 0.4 nm. The smooth surface of the thin crystals ensures the intimate contact between the crystal and the conformal electrode, which is one key to determine the performance of OFETs.25
image file: c7tc04313a-f3.tif
Fig. 3 Conformal rubrene SCFETs on a plane. (a) Schematic and the corresponding cross-sectional diagrams of a device. (b) Optical microscopy images of a real rubrene SCFET array and the magnified view of a single device. (c and d) Typical SEM and AFM images of flexible rubrene nanobelts. The rubrene nanobelt is thin enough so that it shows a bent morphology. (e and f) Typical transfer and output characteristics of the device measured in air at room temperature. The device shows excellent operational stability with zero hysteresis. (g) Distribution of mobility obtained in the saturation region measured from 38 devices.

Fig. 3e and f show the typical transfer and output characteristics of the rubrene SCFETs, respectively. All transistors show clear p-type characteristics with well-defined linear and saturation regimes. Based on such photolithography-compatible electrodes, the device yield is as high as 92%. The fabricated devices exhibit an excellent operational stability with zero hysteresis, and all of the electrical measurements were performed under ambient conditions. The linear behavior at a low VSD in the output curves suggests a good electrode contact.13 The device exhibits an excellent transistor performance at relatively low operating voltages of 10 V. The small operating voltage is very important for lowered power dissipation, improved portability and security. The statistical field-effect mobility (μ) values from 38 devices are shown in the histogram of Fig. 3g. 71% of the transistors show a mobility higher than 5 cm2 V−1 s−1. The maximum mobility value in our bottom-contact rubrene SCFETs on the planar support reaches 24.5 cm2 V−1 s−1 (Fig. S3, ESI), which is higher than the reported highest values of rubrene SCFETs (20 cm2 V−1 s−1),26 including those top-contact rubrene SCFETs. In addition to organic micro/nano single crystals, our conformal electrodes with full flexibility can also be applied for rigid large crystals, and the device shows an excellent field-effect performance as shown in Fig. S4 (ESI).

To examine the conformal capability of the fabricated organic SCFETs, we attached them to different shaped objects, such as a one-dimensional (1D) flexible banknote, a two-dimensional (2D) curved cylindrical pencil, and a three-dimensional (3D) curved glass sphere, and measured their field-effect performances. As shown in Fig. S5 (ESI), the conformal single-crystal FET was fabricated by successively laminating the electrode and the rubrene nanobelt onto a curved surface to effectively avoid the detached phenomenon between the crystal and the electrode, and reduce various types of strain damage on the crystal in the deformation of the whole device. Fig. 4a–c shows the digital photos of the real devices and the corresponding transistor characteristics, which are the best device performances with the highest mobility on different shaped objects. On the banknote, the pencil, and the 0.7 cm glass sphere, the highest mobility is 25.2, 24.2, and 21.4 cm2 V−1 s−1, respectively. They show a near-zero threshold voltage at 0.62, −0.47, and −0.14 V, respectively. The normalized subthreshold swing Si is extremely low (0.17–0.28 V nF decade−1 cm−2), and is one order of magnitude lower than that of the previously reported organic single-crystal devices.26,27 The ultralow subthreshold swing and threshold voltage ensure the low operating voltage. Different from our successively laminating method, we also examined the conformal capability of the whole device under extreme bending, and measured its field-effect performance on a glass capillary with a diameter of 0.13 cm. The device can still normally operate and maintains the typical p-type field-effect performance on the extremly thin glass capillary (Fig. S6, ESI). Our fabricated devices show outstanding transistor characteristics on the static curved objects, indicating the promising potential for future conformal electronics and wearable electronics.


image file: c7tc04313a-f4.tif
Fig. 4 Photographs, optical microscopy images, and electrical properties of the conformal rubrene SCFETs on different shaped objects. (a) On a banknote, the mobility is up to 25.2 cm2 V−1 s−1 at VSD = −20 V. (b) On a pencil with a diameter of 1 cm, the mobility is as high as 24.2 cm2 V−1 s−1 at VSD = −20 V. (c) On a glass sphere with a diameter of 0.7 cm, the mobility is as high as 21.4 cm2 V−1 s−1 at VSD = −20 V.

Elimination of device differences

Device differences are a well-known challenging problem in organic electronics. The organic thin films introduce the effect of grain boundaries that are not appropriate for the study of the basic properties of materials and devices, while for single-crystal devices it is difficult to eliminate the crystal differences such as thickness, length and width differences. As a result, the device differences make it difficult to compare and evaluate the electrical properties of the different organic materials and devices.14,28,29 Here, our photolithography-compatible electrode can produce complex and precise electrode patterns, and can integrate the multiple devices into a single crystal, which is favorable for the realization of uniform devices and the elimination of device differences. As shown in Fig. 5a, based on an evenly spaced electrode pattern, a group of rubrene SCFETs with the same channel length were integrated into a single-crystal nanobelt with uniform width and thickness. According to the corresponding transfer curves in Fig. 5b, the calculated mobility of 6 devices is almost unchanged (Fig. 5c). The average mobility is 16.01333 cm2 V−1 s−1 with a tiny deviation of only 0.51508 cm2 V−1 s−1. The uniform transistor characteristics demonstrate that the conformal electrodes offer reliable and similar contact for rubrene SCFETs, which is beneficial to compare the device performances for revealing the intrinsic properties.
image file: c7tc04313a-f5.tif
Fig. 5 Elimination of device differences for the studies of the fundamental properties of organic transistors. Optical microscopy images and electrical properties of multiple SCFETs based on a rubrene single crystal for (a–c) a fixed channel length at 50 μm, and (d–f) changed channel lengths.

As a typical example, we further studied the relationship between channel length and field-effect mobility based on different spaced electrode patterns. Fig. 5d shows a group of rubrene SCFETs with channel length systematically increasing from 10 to 90 μm at 10 μm steps. The corresponding transfer curves show a regular change shifting towards the negative direction in Fig. 5e. The devices with longer channel lengths were also measured based on the same device group as shown in Fig. S7 (ESI). We observe that, for the same crystal, the field-effect mobility increases with increasing channel length, with negligible change at L > 90 μm after which the changes are not pronounced (Fig. 5f). This trend can be explained by considering the total resistance, Rt = Rc + Rch, as a function of the contact resistance Rc and the channel resistance Rch. Rch changes with channel dimensions and gate voltage VG according to the formula:30

image file: c7tc04313a-t1.tif
Therefore, at fixed VG, the mobility changes with the channel length because of the relative increased magnitude of Rch and the weakened influence of Rc on mobility. The intrinsic channel mobility dominates the transport at longer channel lengths while the contact resistance limits it at shorter channel lengths.31 In addition, the channel width also affects the mobility as shown in Fig. S8 (ESI). At a fixed channel length, the mobility shows an obvious decrease with an increase in the channel width due to the limit of Rc at larger channel widths.

Nondestructive interface contact

In our experiments, the excellent transistor characteristics on both planar and curved surfaces confirm the excellent interface contact between rubrene and PDMS, since interface quality is well known to be extremely sensitive for the field-effect performance.32 To further confirm a good and nondestructive contact between the rubrene crystal and the conformal electrode, we repeatedly laminated the crystal to the electrode and tested their field-effect performance as shown in Fig. 6. First, when one end of one crystal contacted with the soft electrode by a mechanical probe, the van der Waals force spontaneously caused the crystal to gradually wet along the electrode surface. Fig. 6a1 is the schematic image of the device, and the optical microscopy image of the real device is shown on top of Fig. 6a2. The corresponding electrical characteristics are shown in Fig. 6a2. Second, the crystal was mechanically peeled from the electrode and then was adhered onto the electrode again (Fig. 6b1). The electrical measurement results show that the device retains extremely high electrical performance (Fig. 6b2), which powerfully confirms that the soft contact between the crystal and the electrode in our experiments is a nondestructive and reversible process. In the process of device fabrication, the possibly induced mechanical damage on the crystal and the electrode in our manipulation process can be negligible. Finally, the crystal was peeled off from the electrode again, and the other crystal was placed onto the same electrode (Fig. 6c1). The electrical measurement shows that such a device still possesses an excellent device performance with the mobility as high as 20.2 cm2 V−1 s−1, which further reveals the nondestructive contact on the electrode. The soft contact between the crystal and the electrode avoids the production of surface defects on the crystal and the electrode, and ensures the intimate contact interface, resulting in the extremely high field-effect performance in our experiments, as shown in Fig. 3 and 4.
image file: c7tc04313a-f6.tif
Fig. 6 Schematic and optical microscopy images showing the reversible contact between the rubrene nanobelt and the electrode, and the corresponding electrical properties of the devices. (a1 and a2) Attaching Nanobelt 1 on an embedded laminated electrode. (b1 and b2) Peeling off Nanobelt 1 and re-attaching it on the same electrode. (c1 and c2) Removing Nanobelt 1 and attaching Nanobelt 2 on the same electrode at VSD = −20 V.

Low contact resistance

The contact resistance between the electrode and organic semiconductors has long been known to have a strong influence on the electrical performance of OFETs.33–35 The extremely high field-effect performance presented here suggests the low contact resistance in our organic SCFETs. To confirm this point, according to the transfer line method,30 we calculated the contact resistance from the multiple transistors with different channel lengths based on one rubrene single-crystal nanobelt as shown in Fig. 7. The uniform transistor characteristics in Fig. 5c demonstrate that the conformal electrodes offer reliable and similar contact for organic SCFETs, which is beneficial to compare the device performances for revealing their intrinsic properties. Fig. 7a and b shows the width-normalized total resistance (Rt) as a function of channel length for different gate voltages, and the contact resistance (Rc) as a function of gate voltage. The total resistance normalized by the channel width (RtW) at the linear regime (VSD = −0.5 V) was then calculated using the following equation:31
image file: c7tc04313a-t2.tif
where Rch is the channel resistance. The estimated Rc ranges from 35 to 70 KΩ cm, which is one order of magnitude lower than the reported bottom-contact rubrene SCFETs.13,36

image file: c7tc04313a-f7.tif
Fig. 7 Contact resistance of conformal rubrene SCFETs. (a) Width-normalized total device resistances (Rt) as a function of channel length for different gate voltages. (b) Width-normalized contact resistance (Rc) as a function of gate voltage.

In our experiments, the small contact resistance is possibly related to the elimination of electrode steps by applying a soft coplanar bottom-contact electrode configuration, which combines with intimate and nondestructive physical contact with rubrene crystals, resulting in the high field-effect mobility for bottom-contact rubrene SCFETs as shown in Fig. 3 and 4. As mentioned in the Experimental section, the Au electrode embedded in PDMS is peeled off from the flat rigid OTS/Si wafer, which produces a smooth and coplanar structure. The eliminated step (Fig. 3a) favors the intimate contact between the crystal and the electrode, which is extremely important for the bottom-contact organic single-crystal transistors.37,38 Compared with inorganic counterparts, the carrier concentration of organic semiconductors is much lower. The charge transport of organic semiconductors mainly depends on the gate-field induced conductive channel located at the semiconductor/dielectric interface. The low electrode step will enhance the modulation effect of gate voltage at the edge of source/drain electrodes near the conductive channel. As shown in Fig. 4a, the current presents an increase as high as two orders of magnitude only in a very small gate voltage range (−0.6 to 0.8 V), showing the dramatic gate modulation effect and hence resulting in high mobility in our organic SCFETs.

Conclusions

In summary, we applied a novel photolithography-compatible conformal electrode for high performance bottom-contact OFETs. Versatile precise electrode patterns can be designed for the fabrication of multiple devices based on a single rubrene single-crystal nanobelt, which effectively eliminates the device differences for studies of the fundamental properties of OFETs. This electrode offers soft, coplanar, nondestructive, and reversible contacts for high-performance bottom-contact organic SCFETs. The resulting rubrene SCFETs show excellent electrical performance with good device uniformity, zero hysteresis, a device yield as high as 92%, and a high field-effect mobility up to 25.2 cm2 V−1 s−1. Even on a glass sphere with a diameter of 0.7 cm, the conformal rubrene device shows a mobility as high as 21.4 cm2 V−1 s−1. Our designed electrode opens a new route to fabricate high-performance bottom-contact organic SCFETs, shows an advantage for intrinsic properties of OFETs, and presents strong potential for manufacturing large-scale high-integration conformal devices and circuits combined with organic thin films.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the NSFC (51322305, 51703020, 61574032, 61376074, and 91233204), the 111 Project (B13013), and the Fundamental Research Funds for the Central Universities (2412017QD008). This project was funded by the China Postdoctoral Science Foundation (2016M601361), the Open Project of Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education (130028696), and the Northeast Normal University Institute of Physics Discipline Construction Projects (111715014).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc04313a

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