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Defect modification to improve field emission of ZnO NRs by coating ultrathin Pt films
Journal of Alloys and Compounds 714 (2017) 198e203
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Defect modification to improve field emission of ZnO NRs by coating ultrathin Pt films Jijun Ding*, Haixia Chen, Dequan Feng, Haiwei Fu College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2017 Received in revised form 9 April 2017 Accepted 21 April 2017 Available online 23 April 2017
Ultrathin platinum (Pt) films are coated on the surface of ZnO NRs grown on Fe alloy substrates and display excellent field emission performance with the turn-on field Eto as low as 0.40 V/mm and the threshold field Ethr down to 2.62 V/mm. By coating ultrathin Pt films on the surface of the ZnO NRs, this not only helps to modify the linear defects on the outer surface of ZnO NRs, but also suppresses random electron emission along the defect directions. At an external electric field, a large amount of electrons are transported to the top of ZnO NRs where a highly localized electric field is caused, which results in enhanced field emission. Besides, ZnO NRs grown on Fe alloy substrates have a low interfacial contact resistance and intend to reduce the barrier between the substrate and the ZnO. Meanwhile, metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. Finally, a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt contribute to the enhanced current emission efficiency. This work may help the development of the practical electron sources and advanced devices based on ZnO field emitters. © 2017 Elsevier B.V. All rights reserved.
Keywords: Pt films ZnO NRs Electron transport Field emission mechanism
1. Introduction ZnO, with wide band gap (~3.37 eV) and large exciton binding energy (~60 meV), is an ideal material for optoelectronic devices [1,2]. It can be applied to laser diodes, ultraviolet lasers, solar cells, thin film transistors, transparent conductive contacts and emitters [3,4]. One-dimension (1D) nanostructures such as ZnO nanowires (NWs), ZnO nanorods (NRs), and ZnO nanocones have attracted considerable interest due to their novel properties and applications. Field emission properties of various 1D ZnO nanostructures have also been explored extensively. Zhang et al. [5] reported ZnO/ZnS hetero-junction nanocone arrays with excellent field emission performance. Chiu et al. [6] found that the turn-on field of ZnO NWs is significantly decreased after Ga doping but is insensitive to the Ga doping contents. Zhang et al. [7] reported that field emission of ZnO nanoneedle arrays have a close relationship with the surface morphologies. To further improve the field emission performance from nanostructured ZnO emitters, different methods are tried, such as
* Corresponding author. E-mail address: [email protected] (J. Ding). http://dx.doi.org/10.1016/j.jallcom.2017.04.226 0925-8388/© 2017 Elsevier B.V. All rights reserved.
exposing to UV [8], optimizing their shape and aspect ratio [9], doping with metals [10,11] and decorating with Au nanoparticles [12]. Surface coating is also another effective method to enhance the field emission of ZnO nanostructures. Marlinda et al. [13] coated ZnO NRs surface with reduced graphene oxide for low turn on field due to low work function. Maiti et al. [14] coated ZnO NW tips with graphene dispersion to increase surface volume ratio for further improving field emission. However, until now, there have been few attempts of Pt films coating ZnO NRs on alloy substrates and concerning their electron emission. This novel nanostructures may find potential applications in high performance field emitters. In addition, the semiconductor nanostructures deposited on alloy substrates provide important technological merits such as the low interfacial resistance and intend to enhance electrical conduction in field emitters, and thus improve current emission efficiency. In this paper, based on Fe alloy substrates have a low interfacial contact resistance and intend to enhance electrical conduction, meanwhile, considering that Pt metal coating can help to produce more electron tunnel from emitter towards the vacuum, ultrathin platinum (Pt) coated ZnO NRs are grown on Fe alloy substrates. Results indicate that the field emission of ZnO NRs is improved
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largely by coating ultrathin Pt films. Based on the experimental results, field emission mechanism of Pt films coated ZnO NRs has been investigated. 2. Experimental
3. Results and discussion Fig. 1 shows XRD patterns of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. For comparison, ZnO, Fe and Pt diffraction peaks from JCPDS standard cards are drawn on the bottom of Fig. 1. ZnO (100), (002), (101), (102) and (103) diffraction peaks and Fe alloy substrate phase are observed in the XRD patterns. As Pt films are coated on ZnO NRs, no Pt phase is obtained, and XRD pattern is hardly changed except Fe diffraction peaks decrease. This indicates that ultrathin Pt films coating don't affect the preferred orientation of ZnO NRs. Fig. 2 presents SEM images of ZnO NRs (a, b) and Pt films coated ZnO NRs (c, d) with different magnifications. ZnO NRs without Pt films exhibit hexagonal structures with similar height and diameter. As the Pt thin films are coated on the ZnO NRs, the top hexagonal structures of the ZnO NRs become blurred and are no longer flat surface, which can be acted as major emission sites. However, the top structures of ZnO NRs can't be changed largely due to ultrathin Pt films coating in the sub-5 nm regime. At the same time, some of the Pt films fall and coat around the ZnO NRs since Pt films are coated on the ZnO NRs using sputtering coating. However, the sputtered Pt films are attached to the surface of the ZnO NRs without changing the inter-NRs spacing of those. Also, the high magnification images reveal that Pt nanoparticles embedded in the
Fig. 1. XRD patterns of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The ZnO, Fe and Pt diffraction peaks from JCPDS standard cards are drawn on the bottom by solid black, magenta and orange lines, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ZnO NRs are in reasonably close contact with the ZnO NRs but do not be joined together to form a bundle. Fig. 3 displays field emission J-E characteristics of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. The Eto value of ZnO NRs and Pt coated ZnO NRs are 3.37 and 0.40 V/mm, and Ethr are 14.00 and 2.62 V/mm, respectively. The field emission J-E characteristics can be expressed by a simplified Fowler-Nordheim (F-N) equation [16]:
J¼
Ab2 E2
F
exp
! 3 BF2 : bE
(1)
The formula can be changed as following:
! 3 2 J Ab BF2 1 ln 2 ¼ ln , ; F b E E
(2)
3
6:83 103 F2 cm1 : K
b¼
(3)
where, J is in the unit of mA/cm2, E is in the unit of V/mm, F is the work function of the emitter, which is 5.3 eV for ZnO. 3 A ¼ 1:54 106 A,eV,V 2 , B ¼ 6:83 109 eV 2 ,V,m1 , and b is the field enhancement factor that is introduced to quantify the enhancement degree of any tip over a flat surface, i.e., b represents the true value of the electric field at the tip compared to its average macroscopic value [17]. Fig. 4 illustrates F-N plots of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The F-N plots of the field emission current from both the ZnO NRs and Pt films coated ZnO NRs display nonlinear relations between ln(J/E2) versus 1/E. They are only at very high electric fields that the F-N relations become nearly linear. In the case of ZnO NRs, this irregular nonlinearity can be ascribed to the nonuniform geometries of the emitters [18]. However, such nonlinearity in the Pt films coated ZnO NRs are frequently related to different enhancement factors within emitters, space charge effect, and nonuniform local field near the tip of =
Metal alloy substrate containing mainly iron (99 at.%) are cut, ground and polished, then ultrasonically clean with acetone and alcohol in sequence for 15 min. Subsequently, argon plasma clean for 10 min before growing ZnO NRs. ZnO NRs coated with ultrathin Pt films are fabricated by the following procedures. Firstly, ZnO seed layers are deposited onto the cleaned metal Fe alloy substrates by the conventional sol-gel and spin-coating technique and annealed at 350 C for 1 h in air. Secondly, ZnO NRs are synthesized by chemical bath deposition (CBD) method. The ZnO seeds are immersed in a 100 ml mixed aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 25 mM) and hexamethylenetetramine (C6H12N6, 25 mM) at 95 C for 3 h, and the surface are polished with the seeds side facing downward. After growth, the samples are washed by deionized water and dried by nitrogen gas. The similarly fabricated processes about ZnO NRs have been reported elsewhere [15]. Finally, ultrathin Pt films are coated on the ZnO NRs using a DC SPI-Module Sputter Coater. The applied voltage and current are fixed at 1 kV and 8 mA, respectively. As the sputtering time is maintained for 3 min, the Pt film thicknesses are estimated in the sub-5 nm regime. X-ray diffraction (XRD) patterns of the ZnO NRs and Pt coated ZnO NRs are studied by using a D/Max-2400 X-ray diffractometer. Scanning electron microscopy (SEM) images of samples are characterized by a FEI Quanta 250 scanning electron microscope. The current density-field strength (J-E) characteristics are estimated under base pressure prior to 105 Pa at room temperature by using a computer-controlled power source with amperometer. A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm, which is adjusted with insulating polyethylene film with a high breakdown voltage and certain thickness before the measurements. PL properties of the samples are measured by a FluoroMax-3 fluorescence spectrometer. All spectra are measured at room temperature in air.
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Fig. 2. SEM images of ZnO NRs (a, b) and Pt films coated ZnO NRs (c, d) with different magnifications.
Fig. 3. Field emission J-E characteristics of ZnO NRs (rhombus line with dot center) and Pt films coated ZnO NRs (solid sphere line) grown on Fe alloy substrates. The turnon field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. Both values largely decrease by surface coating Pt films, which corresponds to improved field emission.
Fig. 4. F-N characteristics of ZnO NRs (rhombus line with dot center) and Pt films coated ZnO NRs (solid sphere line) deposited on Fe alloy substrates. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)1/E plots, for ZnO NRs and Pt coated ZnO NRs, respectively.
emitter [19]. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)-1/E plots, for ZnO NRs and Pt films coated ZnO NRs, respectively. Combined lower turn-on field Eto and the threshold field Ethr with enhanced field enhancement factor b, indicating that the field emission of ZnO NRs is improved largely by surface coating Pt films. For comparison, Table 1 lists the key parameters of the metal or semiconductor coated ZnO field emitters reported in the literature and in this work. We found that the turn-on field Eto value of the Pt films coated ZnO NRs is smaller than that of both Au nanoparticles
decorated ZnO nanoarrays [13] and carbon nanotubes coated ZnO nanoflowers [20]. Compared with Au nanoparticles decorated ZnO nanopillars [12] and nanoarrays [13], Pt films coated ZnO NRs have a larger field enhancement factor b. To our interesting, all the key parameters, including turn-on field Eto, threshold field Ethr values and field enhancement factor b, are close to that of graphene coated ZnO NWs [14]. So Pt films can be used as coating layer to effectively improve the field emission of ZnO nanostructures. The mechanism of electron emission can be explained as following. Due to ultrathin Pt films are coated on ZnO NRs, field
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Table 1 Key parameters of the metal or semiconductor coated ZnO field emitters reported in the literature and in this work. Field emitters
Eto (V/mm)
Ethr (V/mm)
b (cm1)
Reference
Au nanoparticles decorated ZnO nanopillars Au nanoparticles decorated ZnO nanoarrays Graphene coated ZnO NWs Carbon nanotubes coated ZnO nanoflowers Pt films coated ZnO NRs
2.65 (at 10 mA/cm2) 3.2 (at 0.1 mA/cm2) 1.8 (at 10 mA/cm2) 0.8 (at 1 mA/cm2) 0.40 (at 1 mA/cm2)
/ / 4.91 (at 1 mA/cm2) 1.1 (at 10 mA/cm2) 2.62 (at 0.1 mA/cm2)
3313 3415 10179 / 9969
[12] [13] [14] [20] This work
emission screening effect is not formed after Pt metal coating. We believe that enhanced field emission from the Pt films coated ZnO NRs structures can take place in the following three possible ways. (1) Fig. 5a and b present the schematic drawing of the ZnO NRs and Pt films coated ZnO NRs structures, respectively. In actual growth conditions of ZnO NRs, intrinsic point defects is inevitable resulting in their imperfect surface. Then, the point defects or small-scale defects in this defective ZnO NRs grow to ultimately form large-scale linear defects of the ZnO NRs. As shown in Fig. 5a, there occur a plenty of linear defects on the outer surface of ZnO NRs. During the electron transport process, these linear defects cause random emission sites along different directions formed on the wall of the ZnO NRs, not on the tip. However, after ultrathin Pt films coating on the ZnO NRs, an electron transport passage is formed (Fig. 5b). This not only helps to modify the linear defects on the outer surface of ZnO NRs, but also suppresses random direction electron emission. At an external electric field, a large amount of electrons are transported to the top region of ZnO NRs causing enhanced field emission. (2) During field emission, the electrons have to overcome the two main barriers including the barrier between the substrate and the ZnO NRs, and the barrier between the ZnO NRs and the vacuum, similar result is reported in metal coated CNT [21]. In our work, ZnO NRs grown on Fe alloy substrates have a low interfacial contact resistance and intend to enhance electrical conduction. The barrier between the substrate and the ZnO NRs is expected to be reduced substantially enhancing the field emission properties. Meanwhile, metal film
can help to promote electron passage from the top of ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. (3) A schottky contact is formed at the interface region and a downward band bending is expected when ZnO semiconductor contacts with Fe metal. So electrons are easily transferred from the Fermi level of Fe alloy substrates to the top ZnO NRs. At a very low electric field, these electrons are excited to the ZnO conduction band. Meanwhile, the Fermi levels between the ZnO and Pt are also well matched, as shown in Fig. 5c. Due to the work function of semiconductor ZnO (5.3 eV) is smaller than metal Pt (5.65 eV), which drives the excited electrons from the ZnO conduction band to the Pt Fermi levels. So electrons can easily tunnel through the full barrier width, causing an enhanced field emission current in Pt films coated ZnO NRs. In summary, the electron emission from Pt films coated ZnO NRs is attributed to the formation of electron transport passage, the decrease of two main barriers, and a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt interface. This work may help the development of the practical electron sources and advanced devices based on ZnO field emitters. In order to further confirm the defect modification after coating ultrathin Pt films on the ZnO NRs, PL spectra of ZnO NRs and Pt films coated ZnO NRs are presented in Fig. 6. Generally, PL spectra of ZnO are related to its crystal quality and defect type and concentration. In our experiment, PL spectrum of ZnO NRs shows a broad UV emission band, several blue emissions and a green emission at 560 nm. However, as the ultrathin Pt films are
Fig. 5. Schematic drawing of the ZnO NRs (a) and Pt films coated ZnO NRs structures (b). Where, the cylinder is ZnO NRs, the curve lines are the linear defects on the outer surface of ZnO NRs, the arrows are the emission current, and the yellow shell is ultrathin Pt films. Schematic diagram of the electron transport from the ZnO NRs to Pt and then tunneling into vacuum (c). Metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. Where, work function is defined as the energy difference from the vacuum level to Fermi level (FFe ¼ 4.5 eV, FZnO ¼ 5.3 eV, FPt ¼ 6.65 eV). (EVAC: vacuum level, EV: the top of the valence band, EC: the bottom of the conduction band, EF: Fermi level, J(E): the emission current density, F: work function). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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electron sources and advanced devices based on ZnO field emitters. Acknowledgement This work is supported by the National Natural Science Foundations of China (Grant No. 11447116), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Doctoral Scientific Research Startup Foundation of Xi'an Shiyou University (Grant No. 2016BS12) and Creative Scientific Research Group of XSYU (Grant No. 2014KYCXTD02). References
Fig. 6. PL spectra of ZnO NRs (a) and Pt films coated ZnO NRs (b) grown on Fe alloy substrates. The squared sparse fill pattern correspond to the defect-related PL emission.
coated on the ZnO NRs, there occur a sharp UV peak at 378 nm. Meanwhile, compared with ZnO NRs, all the intensity of the visible PL emission distinctly decrease. The origin of PL emission in ZnO has been investigated for a long time. Generally, UV peak at 378 nm originates from the recombination of the free excitons. The researchers consistently believed that visible emissions come from various intrinsic defects in ZnO. Such as, in most case, blue emission is attributed to interstitial zinc defect level [22]. Also, other hypotheses include ionized ZnO [23] and zinc vacancy [24]. Green emission is consistently ascribed to oxygen vacancies [25]. In this work, the change of PL spectra indicate that crystal quality is improved and intrinsic defects in ZnO NRs is modified after coating ultrathin Pt films on the ZnO NRs. This further confirms that the linear defects on the outer surface of ZnO NRs are modified ensuring highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which is consistent with the discussion about enhanced field emission in Pt films coated ZnO NRs. 4. Conclusions Ultrathin Pt coated ZnO NRs have been grown on Fe alloy substrates and display excellent field emission performance with the turn-on field Eto as low as 0.40 V/mm and the threshold field Ethr down to 2.62 V/mm. The F-N plots of the field emission current from both the ZnO NRs and Pt films coated ZnO NRs display nonlinear relations between ln(J/E2) versus 1/E. They are only at very high electric fields that the F-N relations become nearly linear. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)-1/E plots, for ZnO NRs and Pt films coated ZnO NRs, respectively. Combined lower turn-on field Eto and the threshold field Ethr with enhanced field enhancement factor b, indicating that the field emission of ZnO NRs is improved largely by surface coating Pt films. Metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. We believe that enhanced field emission from Pt films coated ZnO NRs is attributed to the formation of electron transport passage, the decrease of two barrier levels, and a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt interface. This work may help the development of the practical
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Defect modification to improve field emission of ZnO NRs by coating ultrathin Pt films Jijun Ding*, Haixia Chen, Dequan Feng, Haiwei Fu College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2017 Received in revised form 9 April 2017 Accepted 21 April 2017 Available online 23 April 2017
Ultrathin platinum (Pt) films are coated on the surface of ZnO NRs grown on Fe alloy substrates and display excellent field emission performance with the turn-on field Eto as low as 0.40 V/mm and the threshold field Ethr down to 2.62 V/mm. By coating ultrathin Pt films on the surface of the ZnO NRs, this not only helps to modify the linear defects on the outer surface of ZnO NRs, but also suppresses random electron emission along the defect directions. At an external electric field, a large amount of electrons are transported to the top of ZnO NRs where a highly localized electric field is caused, which results in enhanced field emission. Besides, ZnO NRs grown on Fe alloy substrates have a low interfacial contact resistance and intend to reduce the barrier between the substrate and the ZnO. Meanwhile, metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. Finally, a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt contribute to the enhanced current emission efficiency. This work may help the development of the practical electron sources and advanced devices based on ZnO field emitters. © 2017 Elsevier B.V. All rights reserved.
Keywords: Pt films ZnO NRs Electron transport Field emission mechanism
1. Introduction ZnO, with wide band gap (~3.37 eV) and large exciton binding energy (~60 meV), is an ideal material for optoelectronic devices [1,2]. It can be applied to laser diodes, ultraviolet lasers, solar cells, thin film transistors, transparent conductive contacts and emitters [3,4]. One-dimension (1D) nanostructures such as ZnO nanowires (NWs), ZnO nanorods (NRs), and ZnO nanocones have attracted considerable interest due to their novel properties and applications. Field emission properties of various 1D ZnO nanostructures have also been explored extensively. Zhang et al. [5] reported ZnO/ZnS hetero-junction nanocone arrays with excellent field emission performance. Chiu et al. [6] found that the turn-on field of ZnO NWs is significantly decreased after Ga doping but is insensitive to the Ga doping contents. Zhang et al. [7] reported that field emission of ZnO nanoneedle arrays have a close relationship with the surface morphologies. To further improve the field emission performance from nanostructured ZnO emitters, different methods are tried, such as
* Corresponding author. E-mail address: [email protected] (J. Ding). http://dx.doi.org/10.1016/j.jallcom.2017.04.226 0925-8388/© 2017 Elsevier B.V. All rights reserved.
exposing to UV [8], optimizing their shape and aspect ratio [9], doping with metals [10,11] and decorating with Au nanoparticles [12]. Surface coating is also another effective method to enhance the field emission of ZnO nanostructures. Marlinda et al. [13] coated ZnO NRs surface with reduced graphene oxide for low turn on field due to low work function. Maiti et al. [14] coated ZnO NW tips with graphene dispersion to increase surface volume ratio for further improving field emission. However, until now, there have been few attempts of Pt films coating ZnO NRs on alloy substrates and concerning their electron emission. This novel nanostructures may find potential applications in high performance field emitters. In addition, the semiconductor nanostructures deposited on alloy substrates provide important technological merits such as the low interfacial resistance and intend to enhance electrical conduction in field emitters, and thus improve current emission efficiency. In this paper, based on Fe alloy substrates have a low interfacial contact resistance and intend to enhance electrical conduction, meanwhile, considering that Pt metal coating can help to produce more electron tunnel from emitter towards the vacuum, ultrathin platinum (Pt) coated ZnO NRs are grown on Fe alloy substrates. Results indicate that the field emission of ZnO NRs is improved
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largely by coating ultrathin Pt films. Based on the experimental results, field emission mechanism of Pt films coated ZnO NRs has been investigated. 2. Experimental
3. Results and discussion Fig. 1 shows XRD patterns of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. For comparison, ZnO, Fe and Pt diffraction peaks from JCPDS standard cards are drawn on the bottom of Fig. 1. ZnO (100), (002), (101), (102) and (103) diffraction peaks and Fe alloy substrate phase are observed in the XRD patterns. As Pt films are coated on ZnO NRs, no Pt phase is obtained, and XRD pattern is hardly changed except Fe diffraction peaks decrease. This indicates that ultrathin Pt films coating don't affect the preferred orientation of ZnO NRs. Fig. 2 presents SEM images of ZnO NRs (a, b) and Pt films coated ZnO NRs (c, d) with different magnifications. ZnO NRs without Pt films exhibit hexagonal structures with similar height and diameter. As the Pt thin films are coated on the ZnO NRs, the top hexagonal structures of the ZnO NRs become blurred and are no longer flat surface, which can be acted as major emission sites. However, the top structures of ZnO NRs can't be changed largely due to ultrathin Pt films coating in the sub-5 nm regime. At the same time, some of the Pt films fall and coat around the ZnO NRs since Pt films are coated on the ZnO NRs using sputtering coating. However, the sputtered Pt films are attached to the surface of the ZnO NRs without changing the inter-NRs spacing of those. Also, the high magnification images reveal that Pt nanoparticles embedded in the
Fig. 1. XRD patterns of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The ZnO, Fe and Pt diffraction peaks from JCPDS standard cards are drawn on the bottom by solid black, magenta and orange lines, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ZnO NRs are in reasonably close contact with the ZnO NRs but do not be joined together to form a bundle. Fig. 3 displays field emission J-E characteristics of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. The Eto value of ZnO NRs and Pt coated ZnO NRs are 3.37 and 0.40 V/mm, and Ethr are 14.00 and 2.62 V/mm, respectively. The field emission J-E characteristics can be expressed by a simplified Fowler-Nordheim (F-N) equation [16]:
J¼
Ab2 E2
F
exp
! 3 BF2 : bE
(1)
The formula can be changed as following:
! 3 2 J Ab BF2 1 ln 2 ¼ ln , ; F b E E
(2)
3
6:83 103 F2 cm1 : K
b¼
(3)
where, J is in the unit of mA/cm2, E is in the unit of V/mm, F is the work function of the emitter, which is 5.3 eV for ZnO. 3 A ¼ 1:54 106 A,eV,V 2 , B ¼ 6:83 109 eV 2 ,V,m1 , and b is the field enhancement factor that is introduced to quantify the enhancement degree of any tip over a flat surface, i.e., b represents the true value of the electric field at the tip compared to its average macroscopic value [17]. Fig. 4 illustrates F-N plots of ZnO NRs and Pt films coated ZnO NRs grown on Fe alloy substrates. The F-N plots of the field emission current from both the ZnO NRs and Pt films coated ZnO NRs display nonlinear relations between ln(J/E2) versus 1/E. They are only at very high electric fields that the F-N relations become nearly linear. In the case of ZnO NRs, this irregular nonlinearity can be ascribed to the nonuniform geometries of the emitters [18]. However, such nonlinearity in the Pt films coated ZnO NRs are frequently related to different enhancement factors within emitters, space charge effect, and nonuniform local field near the tip of =
Metal alloy substrate containing mainly iron (99 at.%) are cut, ground and polished, then ultrasonically clean with acetone and alcohol in sequence for 15 min. Subsequently, argon plasma clean for 10 min before growing ZnO NRs. ZnO NRs coated with ultrathin Pt films are fabricated by the following procedures. Firstly, ZnO seed layers are deposited onto the cleaned metal Fe alloy substrates by the conventional sol-gel and spin-coating technique and annealed at 350 C for 1 h in air. Secondly, ZnO NRs are synthesized by chemical bath deposition (CBD) method. The ZnO seeds are immersed in a 100 ml mixed aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 25 mM) and hexamethylenetetramine (C6H12N6, 25 mM) at 95 C for 3 h, and the surface are polished with the seeds side facing downward. After growth, the samples are washed by deionized water and dried by nitrogen gas. The similarly fabricated processes about ZnO NRs have been reported elsewhere [15]. Finally, ultrathin Pt films are coated on the ZnO NRs using a DC SPI-Module Sputter Coater. The applied voltage and current are fixed at 1 kV and 8 mA, respectively. As the sputtering time is maintained for 3 min, the Pt film thicknesses are estimated in the sub-5 nm regime. X-ray diffraction (XRD) patterns of the ZnO NRs and Pt coated ZnO NRs are studied by using a D/Max-2400 X-ray diffractometer. Scanning electron microscopy (SEM) images of samples are characterized by a FEI Quanta 250 scanning electron microscope. The current density-field strength (J-E) characteristics are estimated under base pressure prior to 105 Pa at room temperature by using a computer-controlled power source with amperometer. A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm, which is adjusted with insulating polyethylene film with a high breakdown voltage and certain thickness before the measurements. PL properties of the samples are measured by a FluoroMax-3 fluorescence spectrometer. All spectra are measured at room temperature in air.
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Fig. 2. SEM images of ZnO NRs (a, b) and Pt films coated ZnO NRs (c, d) with different magnifications.
Fig. 3. Field emission J-E characteristics of ZnO NRs (rhombus line with dot center) and Pt films coated ZnO NRs (solid sphere line) grown on Fe alloy substrates. The turnon field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. Both values largely decrease by surface coating Pt films, which corresponds to improved field emission.
Fig. 4. F-N characteristics of ZnO NRs (rhombus line with dot center) and Pt films coated ZnO NRs (solid sphere line) deposited on Fe alloy substrates. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)1/E plots, for ZnO NRs and Pt coated ZnO NRs, respectively.
emitter [19]. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)-1/E plots, for ZnO NRs and Pt films coated ZnO NRs, respectively. Combined lower turn-on field Eto and the threshold field Ethr with enhanced field enhancement factor b, indicating that the field emission of ZnO NRs is improved largely by surface coating Pt films. For comparison, Table 1 lists the key parameters of the metal or semiconductor coated ZnO field emitters reported in the literature and in this work. We found that the turn-on field Eto value of the Pt films coated ZnO NRs is smaller than that of both Au nanoparticles
decorated ZnO nanoarrays [13] and carbon nanotubes coated ZnO nanoflowers [20]. Compared with Au nanoparticles decorated ZnO nanopillars [12] and nanoarrays [13], Pt films coated ZnO NRs have a larger field enhancement factor b. To our interesting, all the key parameters, including turn-on field Eto, threshold field Ethr values and field enhancement factor b, are close to that of graphene coated ZnO NWs [14]. So Pt films can be used as coating layer to effectively improve the field emission of ZnO nanostructures. The mechanism of electron emission can be explained as following. Due to ultrathin Pt films are coated on ZnO NRs, field
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Table 1 Key parameters of the metal or semiconductor coated ZnO field emitters reported in the literature and in this work. Field emitters
Eto (V/mm)
Ethr (V/mm)
b (cm1)
Reference
Au nanoparticles decorated ZnO nanopillars Au nanoparticles decorated ZnO nanoarrays Graphene coated ZnO NWs Carbon nanotubes coated ZnO nanoflowers Pt films coated ZnO NRs
2.65 (at 10 mA/cm2) 3.2 (at 0.1 mA/cm2) 1.8 (at 10 mA/cm2) 0.8 (at 1 mA/cm2) 0.40 (at 1 mA/cm2)
/ / 4.91 (at 1 mA/cm2) 1.1 (at 10 mA/cm2) 2.62 (at 0.1 mA/cm2)
3313 3415 10179 / 9969
[12] [13] [14] [20] This work
emission screening effect is not formed after Pt metal coating. We believe that enhanced field emission from the Pt films coated ZnO NRs structures can take place in the following three possible ways. (1) Fig. 5a and b present the schematic drawing of the ZnO NRs and Pt films coated ZnO NRs structures, respectively. In actual growth conditions of ZnO NRs, intrinsic point defects is inevitable resulting in their imperfect surface. Then, the point defects or small-scale defects in this defective ZnO NRs grow to ultimately form large-scale linear defects of the ZnO NRs. As shown in Fig. 5a, there occur a plenty of linear defects on the outer surface of ZnO NRs. During the electron transport process, these linear defects cause random emission sites along different directions formed on the wall of the ZnO NRs, not on the tip. However, after ultrathin Pt films coating on the ZnO NRs, an electron transport passage is formed (Fig. 5b). This not only helps to modify the linear defects on the outer surface of ZnO NRs, but also suppresses random direction electron emission. At an external electric field, a large amount of electrons are transported to the top region of ZnO NRs causing enhanced field emission. (2) During field emission, the electrons have to overcome the two main barriers including the barrier between the substrate and the ZnO NRs, and the barrier between the ZnO NRs and the vacuum, similar result is reported in metal coated CNT [21]. In our work, ZnO NRs grown on Fe alloy substrates have a low interfacial contact resistance and intend to enhance electrical conduction. The barrier between the substrate and the ZnO NRs is expected to be reduced substantially enhancing the field emission properties. Meanwhile, metal film
can help to promote electron passage from the top of ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. (3) A schottky contact is formed at the interface region and a downward band bending is expected when ZnO semiconductor contacts with Fe metal. So electrons are easily transferred from the Fermi level of Fe alloy substrates to the top ZnO NRs. At a very low electric field, these electrons are excited to the ZnO conduction band. Meanwhile, the Fermi levels between the ZnO and Pt are also well matched, as shown in Fig. 5c. Due to the work function of semiconductor ZnO (5.3 eV) is smaller than metal Pt (5.65 eV), which drives the excited electrons from the ZnO conduction band to the Pt Fermi levels. So electrons can easily tunnel through the full barrier width, causing an enhanced field emission current in Pt films coated ZnO NRs. In summary, the electron emission from Pt films coated ZnO NRs is attributed to the formation of electron transport passage, the decrease of two main barriers, and a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt interface. This work may help the development of the practical electron sources and advanced devices based on ZnO field emitters. In order to further confirm the defect modification after coating ultrathin Pt films on the ZnO NRs, PL spectra of ZnO NRs and Pt films coated ZnO NRs are presented in Fig. 6. Generally, PL spectra of ZnO are related to its crystal quality and defect type and concentration. In our experiment, PL spectrum of ZnO NRs shows a broad UV emission band, several blue emissions and a green emission at 560 nm. However, as the ultrathin Pt films are
Fig. 5. Schematic drawing of the ZnO NRs (a) and Pt films coated ZnO NRs structures (b). Where, the cylinder is ZnO NRs, the curve lines are the linear defects on the outer surface of ZnO NRs, the arrows are the emission current, and the yellow shell is ultrathin Pt films. Schematic diagram of the electron transport from the ZnO NRs to Pt and then tunneling into vacuum (c). Metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. Where, work function is defined as the energy difference from the vacuum level to Fermi level (FFe ¼ 4.5 eV, FZnO ¼ 5.3 eV, FPt ¼ 6.65 eV). (EVAC: vacuum level, EV: the top of the valence band, EC: the bottom of the conduction band, EF: Fermi level, J(E): the emission current density, F: work function). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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electron sources and advanced devices based on ZnO field emitters. Acknowledgement This work is supported by the National Natural Science Foundations of China (Grant No. 11447116), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Doctoral Scientific Research Startup Foundation of Xi'an Shiyou University (Grant No. 2016BS12) and Creative Scientific Research Group of XSYU (Grant No. 2014KYCXTD02). References
Fig. 6. PL spectra of ZnO NRs (a) and Pt films coated ZnO NRs (b) grown on Fe alloy substrates. The squared sparse fill pattern correspond to the defect-related PL emission.
coated on the ZnO NRs, there occur a sharp UV peak at 378 nm. Meanwhile, compared with ZnO NRs, all the intensity of the visible PL emission distinctly decrease. The origin of PL emission in ZnO has been investigated for a long time. Generally, UV peak at 378 nm originates from the recombination of the free excitons. The researchers consistently believed that visible emissions come from various intrinsic defects in ZnO. Such as, in most case, blue emission is attributed to interstitial zinc defect level [22]. Also, other hypotheses include ionized ZnO [23] and zinc vacancy [24]. Green emission is consistently ascribed to oxygen vacancies [25]. In this work, the change of PL spectra indicate that crystal quality is improved and intrinsic defects in ZnO NRs is modified after coating ultrathin Pt films on the ZnO NRs. This further confirms that the linear defects on the outer surface of ZnO NRs are modified ensuring highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which is consistent with the discussion about enhanced field emission in Pt films coated ZnO NRs. 4. Conclusions Ultrathin Pt coated ZnO NRs have been grown on Fe alloy substrates and display excellent field emission performance with the turn-on field Eto as low as 0.40 V/mm and the threshold field Ethr down to 2.62 V/mm. The F-N plots of the field emission current from both the ZnO NRs and Pt films coated ZnO NRs display nonlinear relations between ln(J/E2) versus 1/E. They are only at very high electric fields that the F-N relations become nearly linear. The estimated field enhancement factor b are 1268 and 9969 cm1, calculated from the slope K of ln(J/E2)-1/E plots, for ZnO NRs and Pt films coated ZnO NRs, respectively. Combined lower turn-on field Eto and the threshold field Ethr with enhanced field enhancement factor b, indicating that the field emission of ZnO NRs is improved largely by surface coating Pt films. Metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. We believe that enhanced field emission from Pt films coated ZnO NRs is attributed to the formation of electron transport passage, the decrease of two barrier levels, and a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt interface. This work may help the development of the practical
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