Vertically-Aligned Single-Crystal Nanocone Arrays: Controlled

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Vertically-Aligned Single-Crystal Nano-cone Arrays: Controlled Fabrication and Enhanced Field Emission Jinglai Duan, Dang Yuan Lei, Fei Chen, Shu Ping Lau, William I. Milne, Maria Eugenia Toimil-Molares, Christina Trautmann, and Jie Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09374 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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ACS Applied Materials & Interfaces

Vertically-Aligned Single-Crystal Nano-cone Arrays: Controlled Fabrication and Enhanced Field Emission Jing Lai Duan,†,‡,‖ Dang Yuan Lei,*,‡,‖Fei Chen,‡ Shu Ping Lau,‡ William I. Milne,§ M.E. Toimil-Molares,⊥ Christina Trautmann,⊥,# Jie Liu*,† †

Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China

‡

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China

§

Department of Engineering, Electrical Engineering Division, University of Cambridge, 9 JJ Thomson Avenue, CB3 0FA , Cambridge , United Kingdom

Materials Research Department, GSI Helmholtz Centre for Heavy Ion Research, 64291 Darmstadt, Germany ⊥

#

Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany

KEYWORDS Ion track template, copper, single-crystal, nano-cone array, field emission

ABSTRACT

Metal nanostructures with conical shape, vertical alignment, large ratio of cone height and curvature radius at the apex, controlled cone angle, and single-crystal structure are ideal

ACS Paragon Plus Environment

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candidates for enhancing field electron-emission efficiency with additional merits such as good mechanical and thermal stability. However, fabrication of such nanostructures possessing all these features is challenging. Here, we report on the controlled fabrication of large scale, vertically aligned, and mechanically self-supported single-crystal Cu nano-cones with controlled cone angle and enhanced field emission. The Cu nano-cones were fabricated by ion-track templates in combination with electrochemical deposition. Their cone angle is controlled in the range from 0.3o to 6.2o by asymmetrically selective etching of the ion tracks and the minimum tip curvature diameter reaches down to 6 nm. The field emission measurements show that the turn-on electric field of the Cu nano-cone field emitters can be as low as 1.9 V/µm at current density of 10 µA/cm2 (a record low value for Cu nanostructures, to the best of our knowledge). The maximum field enhancement factor we measured was as large as 6068, indicating that the Cu nano-cones are promising candidates for field emission applications.

1. INTRODUCTION Field emission, also known as field electron emission, is a fundamental physical phenomenon associated with quantum tunneling, in which electrons below or close to the Fermi level escape from the surface of materials by tunneling through surface potential barrier lowered with the aid of an external electric field.1 The field electron emission phenomenon has increasingly attracted research attention because of its potential importance in applications such as high-power vacuum electronic devices, microwave-generation devices, flat panel displays, vacuum micro- or nanoelectronic devices involved in the fields of consumer goods, military technology, and space technology.2-4 These applications usually require field emitters having, some or all of the following characteristics: low turn-on field, high current density, long-term emission stability, miniaturized size, and large scale. To achieve a low turn-on field and a high current density, field


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emitter materials should preferably have a low work function ( or electron affinity) and be nanostructured with needle-like shapes to significantly enhance the field at the needle tips.3,4 The field enhancement factor β is directly related to the tip height and inversely proportional to the tip radius.3 It is thus important to make the needle tips as sharp as possible in order to reduce the applied external field required for turning on the electron emission.3-6 To date, several metal, semiconductor and carbon nanostructures have been developed in an attempt to achieve better field emission performance, including Ge,7 Cu,8 and Au nano-wires,9 PrB6 nano-rods,10 WS2 nano-tubes,11 ZnO nano-walls,12 graphene nanoedges,13 boron and Cu nano-cones,14,15 carbon nanotubes and many others.16-22 Among these nanostructures, nanocones have shown unique advantages, including good mechanical stability, large ratio of cone height and curvature radius at the apex, low turn-on field, high emission current density, and large field-enhancement factor. Excellent examples have also been demonstrated using wide band-gap semiconducting ZnO and metallic nickel nanostructures.5,23,24 In the past decade, several fabrication methods have been proposed to fabricate nano-cones, including ion-beam sputtering,25 reactive ion etching,26 plasma etching,27 nanotransfer printing,28 chemical vapor deposition,29 templating methods,15 wet chemical methods,30 and electrochemical deposition.5 Although these various methods of fabrication have shown some success in the preparation of conical field emitters, issues associated with relatively low height (shorter than 5 µm),30 random or only partial vertical alignment,6,30 field screening due to high density and large cone angle,5,6,26 and large scatter in the height27 still remain unsolved. In this work, we report on the controlled fabrication of large scale, vertically aligned, and mechanically self-supported single-crystal Cu nano-cone arrays with controlled cone angle leading to enhanced field emission. The Cu nano-cones were fabricated by ion-track templates in


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combination with electrochemical deposition. Their cone angle can be controlled in the range from 0.3o to 6.2o by asymmetrical selective etching of the ion tracks and the minimum tip curvature diameter reaches down to 6 nm. Field emission measurements show that the turn-on electric field of the Cu nano-cone field emitters can be as low as 1.9 V/µm defined at a current density of 10 µA/cm2 (a record low value for Cu nanostructures, to the best of our knowledge). The maximum field enhancement factor we measured was as large as 6068, indicating that the Cu nano-cones are promising candidates for field emission applications. Finite Element Method (FEM) simulations reveal that the enhanced field tends to localize at the sharpest site (top of the rounded tip and edge of the flat tip). FEM simulations also suggest that a lower turn-on field and larger field enhancement factor would be expected by further minimizing tip size of nano-cones.

2. EXPERIMENTAL SECTION Cu nano-cones were fabricated by electrochemical deposition in conical channels of ion-track templates.15 First, 30-µm thick polycarbonate (PC) foils (Makrofol N, Bayer Leverkusen) were irradiated under normal beam incidence with 9.5 MeV per nucleon (MeV/u) 209Bi ions at the Heavy Ion Research Facility at Lanzhou (HIRFL) and with 11.1 MeV/u 238U ions at the UNILAC linear accelerator of GSI at Darmstadt. According to the Monte-Carlo simulations using SRIM 2013, the electronic energy loss and projected range of 9.5 MeV/u 209Bi in polycarbonate are 1.4 keV/Å and 146 µm, respectively. In the case of 11.1 MeV/u 238U, they are 1.6 keV/Å and 165 µm, respectively. The simulations show that both ion species have very similar energy losses in polycarbonate and thus are sufficient to obtain a homogeneous track etching. The electronic energy loss and the projected range in polycarbonate of both ion species are sufficiently high and therefore are suitable for this work. At both facilities, the irradiation


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fluence was 8×105 ions/cm2. Conical nanopores were obtained by selectively etching the ion tracks from one side. Various etchants composed of 9 M NaOH and methanol in various volume ratios were employed. Depending on the volume ratio of NaOH, the etching times are 7 minutes (100% NaOH), 7.5 minutes (90% NaOH), 8.5 minutes (70% NaOH), 11 minutes (50% NaOH), 19.5 minutes (30% NaOH), and 79 minutes (5% NaOH). Subsequently, Cu nano-cones were electrochemically deposited in the channels at 50 °C by applying a constant voltage of 200 mV. The electrolyte consisted of 75 g/l CuSO4⋅5H2O and 30 g/l H2SO4. A thin sputtered Au layer reinforced by an electroplated Cu layer was used as cathode for the Cu deposition. After deposition, the host PC template was dissolved by immersing the sample in dichloromethane. The morphology of the nano-cones was analyzed by scanning electron microscopy (SEM, Philips XL30 and FEI Nova NanoSEM 450). For TEM characterizations, we put nano-cone samples in dichloromethane to dissolve polycarbonate for several times and then use sonication to detach cones from supporting substrates which were remained in dichloromethane solution. After detaching the cones from the supporting substrate using an ultrasonic field, the microstructure of the cones was investigated by high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN). The field emission characteristics of nanocone arrays were measured in a vacuum chamber with a base pressure below 1×10-5 Pa. The measurements were conducted on a standard parallel-plate-electrode configuration where a stainless-steel plate was used as anode, and each sample (1/4 of ɸ 6 mm, ~ 7.1 mm2) was carefully fixed onto a flat copper stage (by means of silver glue) which served as cathode. The distance between the nano-cone tips and the anode was 300 µm. The I-V (current-voltage) curves were recorded by LabVIEW programming through a Keithley 248 power source with a computer-controlled data-acquisition card.


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3. RESULTS AND DISCUSSION The main fabrication procedures employed are schematically illustrated in Figure 1. Briefly, a polycarbonate (PC) foil is irradiated with swift heavy ions that create ion tracks along their trajectory (step 1). According to SRIM calculation, the electronic energy losses in polycarbonate of both ions are around 1.5 keV/Å. Such high energy losses are sufficient to create continuous ion tracks and the registration rate of ions can be 100%. Thus the areal density of the ion tracks can be controlled by adjusting irradiation fluence. This step is followed by sputtering of a thin gold layer onto one surface of the PC foil which serves as the cathode for the subsequent asymmetric etching of ion tracks (step 2). The ion tracks are selectively etched from the uncoated side of the film. This asymmetric etching (step 3) results in conical pores that are subsequently filled with Cu to form Cu nano-cones using electrochemical deposition (step 4). During etching, the film was placed between two cells and only the cell facing the uncovered side of the film is filled with the etchant. The etching process is monitored by measuring the electrical current flowing across the film when a constant voltage of 0.2 V is applied. After pore filling, the deposition is extended to form a planar closed film as a support layer for the nano-cones (step 5). Finally the template matrix is removed using dichloromethane, leaving vertically aligned nanocones standing on the Cu film. This method has unique advantages and high flexibility for the fabrication of nano-cone arrays because key parameters including electrodeposited material, cone height, cone angle, and areal density can be independently controlled. In addition, costs of the nano-cone arrays based on ion track method are acceptable and high producibility is available because some accelerators, e.g. Heavy Ion Research Facility at Lanzhou (HIRFL), have commercial beam-lines which supports mass production of Nuclear Track Membranes (NTM).


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Figure 1. Schematics of main steps employed in the fabrication of Cu nano-cones based on the combination of ion track etching and electrochemical deposition. 1) ion irradiation of polycarbonate foil; 2) sputtering of thin gold film; 3) asymmetric etching to form conical pores; 4) electrochemical deposition of Cu in conical pores; 5) over electrodeposition to form substrate; 6) free-standing nano-cone array after removal of template. Figure 2a shows a representative SEM micrograph of the surface of a PC template with conical pores of similar base diameter ~ 4 µm. The pores are well separated and randomly distributed due to the low fluence applied when irradiating the polymer foil. The track etching is produced by two competing etching rates, namely anisotropic etching along the ion track at rate vt and isotropic etching at the bulk etching rate vb of the undamaged polymer matrix,31 as schematically shown in the inset of Figure 2a. The pore shape is determined by the ratio of the two etching rates. A cylindrical shape (often used to prepare nano-wires32,33) is obtained when vt is much larger than vb and a conical shape is obtained when vt is comparable to vb. During etching of the tracks, vt is very sensitive to the concentration of NaOH because the radicals produced by ion irradiation can be selectively etched whereas vb is relatively insensitive to this and thus a cylindrical shape is frequently formed. In this work, methanol is added to the 9M NaOH etchant because methanol attacks bulk polycarbonate by a depolymerization process and


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thus enhances vb without significantly influencing vt. After filling the conical pores with Cu by electrochemical deposition the Cu nano-cone arrays are formed. A typical low-magnification SEM micrograph of nano-cone arrays after template dissolution is displayed in Figure 2b. The nano-cones are homogeneously distributed on the substrate over a large scale indicating that all pores were nearly completely filled. Note that unfilled pores would give rise to some small protrusions on the substrate (see step 4 in Figure 1). For comparison, a SEM image of a partially filled sample (2.9°, 5×105 cm-2) is shown in Figure S1, Supporting Information.

Figure 2. SEM micrographs of (a) PC template with conical pores and (b) Cu nano-cone arrays. The inset in (a) shows a schematic cross section of a conical pore indicating the track etching rate vt and bulk etching rate vb.


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Figure 3. SEM micrographs of six Cu nano-cones with various cone angles. The cone angle increases with increasing methanol concentration in the etchant. Each cone angle is an average value extracted from statistics over 20 nano-cones and the standard deviation is less than 5% for all samples.

By changing the volume ratio of methanol to 9 M NaOH, we tailored the cone angle of the conical pores and subsequently of the Cu nano-cones. Figure 3 shows SEM images of Cu nanocones with different cone angles. All nano-cones show smooth, homogeneous contours along their lengths. The tip diameter in this instance is about 50 nm and the length is nearly 28 µm which is slightly thinner than the template thickness. By increasing the volume ratio of methanol to 9 M NaOH from 0 to 95%, the cone angle gradually increases from 0.3º to 6.2º. Here each cone angle is an average value extracted from statistics over 20 nano-cones and the standard deviation is less than 5% for all samples. An example of statistics for the Cu nano-cones with cone angle of 1.7° is shown in Figure S2 in the Supporting Information. It is also noteworthy to


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mention that, the cone angle is well defined by the volume ratio of methanol to 9 M NaOH. Other parameters such as etching temperature, quality of polycarbonate films, and purity of chemicals are possible factors which can influence the ultimate geometry. In this work, the etching temperature was fixed (22 ± 1 °C). A high quality of polycarbonate films (Makrofol N, Bayer Leverkusen) were used. All chemicals used were of analytical grade. Therefore the geometry is highly reproducible and we can thus gradually change the cone angle by changing the volume ratio of methanol to 9 M NaOH of etchant.

Figure 4. Representative SEM images of nano-wire (a) and five Cu nano-cone arrays with cone angles of 0.3° (b), 1.7° (c), 2.2° (d), 2.9° (e), and 6.2° (d).


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To achieve enhanced and stable field emission, the emitters should be mechanically robust and provide a good electrical contact to the surface. Figure 4 displays SEM images of nano-cone arrays with different cone angles. For comparison, a SEM image of short nano-wire array is shown in Figure 4a. For a cone angle of 0.3° (Figure 4b), the nano-cones show very weak mechanical strength as previously observed for nano-wires.34 In Figure 4b, a large part of the nano-cones collapse, aggregate, and support each other due to the small sizes of both cone tip and base. The mechanism of aggregation has been proposed in our previous work.34 In addition, other cones completely fall down and lie on the substrate. Such a geometrical configuration would limit the field emission due to, the reduced height and smaller number of emitters. When the cone angle reaches 1.7° (Figure 4c), a larger amount of nano-cones are well separated and vertically aligned. The amount of stable cones increases further when the cone angle reaches 2.2o (Figure 4d). Finally, the arrays of nano-cones with cone angles 2.9o and 6.2o (Figure 4d) exhibit the best mechanical stability.


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Figure 5. Representative TEM images of nano-cones with cone angle of 2.2°, (a) round and (b) flat tips. The insets in (a) show the HRTEM images captured at the round tip (upper left) and at a middle area of the cone (lower right). The transmission electron microscopy (TEM) images shown in Figure 5 reveal that the nanocones have a perfect conical shape and are extremely straight despite a sharp decrease in the cone


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diameter at the proximity of the nano-cone apex (see the inset of Figure 5a). It should be pointed out that an ultrasonic field was used during sample preparation for TEM characterization, corroborating the mechanical strength of the fabricated structures. The high resolution TEM images shown as insets in Figure 5a show the single-crystallinity of the nano-cone tip (upper left inset) and of the middle section of the cone (lower right inset). Over several micrometers along the cone’s length, no grain boundaries were observed. In addition to the conical geometry, the reduced presence of defects such as voids, grain boundaries, and dislocations in single-crystalline emitters is expected to further improve their mechanical strength. Note that another key parameter of the nano-cone structures is the extremely small tip curvature diameter (~6 nm), demonstrating a new lower size limit of nanostructures prepared by ion-track templates so far (the reported thinnest diameter of nano-wires prepared using the same ion track-based method is 9-10 nm).33 In addition, we observed two different tip shapes, namely rounded (Figure 5a) and flat (Figure 5b). The formation of different tip shapes may be due to slightly different crystallographic orientations. Thanks to their sharp tips, large heights, vertical alignment, and single-crystal structure, these Cu nano-cones are excellent candidates to be applied in field electron emission and electronic devices. To assess this, the current-voltage (I-V) characteristics of the Cu nano-cones with cone angle of 6.2° and 2.9° were measured, in a parallel-plate-electrode configuration. The field emission properties of nano-cones with cone angles of 0.3°, 0.7°, 1.7°, and 2.2° were not measured because of two reasons. Firstly, the mechanically unstable nano-cones are not suitable for field emission applications and the cones with cone angles of 0.3°, 0.7°, 1.7°, and 2.2° are not mechanically robust, as revealed by the SEM images shown in Figure 4. Secondly, an uncertain number of cones in the samples with cone angles of 0.3°, 0.7°, 1.7°, and 2.2° fell down. This


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uncertainty of field emitter number makes difficult to compare their field emission results directly. For comparison, the field emission properties of an array of cylindrical Cu nano-wire arrays of 75 nm in diameter, 2 µm in length, and 5×108 cm-2 in areal density were also measured. The nano-wires are vertically aligned and mechanically robust, as shown in Figure 4a. Note that the nano-wires were electrochemically deposited at 100 mV at 50 °C and they possess single crystal structure as reported in our previous work.35 The I-V curves of macroscopic applied field E versus emission current density J of the aforementioned three samples are presented in Figure 6a-c. It should be addressed here that, for field emitters, conditioning effects also influence their field emission performance and usually result in hysteresis in I-V curves. In our FE measurements, field emission hysteresis was also observed for the first two rounds of voltage ramping and it became less noticeable afterwards. The hysteresis of our samples is possibly due to gas adsorption and desorption on nano-cone tips. As generally adopted,3,19 the turn-on field is defined as the field at which the current density reaches 10 µA/cm2. As seen from Figure 6d, both nano-cone samples show significantly lower turn-on fields than the nano-wires. In particular, the Cu nano-cone sample with cone angle of 2.9° shows the best field emission properties with the lowest turn-on field of 1.9 V/µm (c.f. 5.0 V/µm for the nano-cone sample with cone angle of 6.2° and 7.0 V/µm for the nano-wire sample). To the best of our knowledge, the turn-on field of 1.9 V/µm is a record low value compared to other values reported for Cu nanostructures, e.g. 12.4 V/µm for nano-pillars, 4.6 V/µm for nano-wires, and 22 V/µm for nano-cones (Table 1). For a comprehensive comparison, Table 1 summarizes the key parameters of field emission, geometry, structure and material for various metallic nanostructure-based field emitters reported in previous literature and in this work. Furthermore, the turn-on field observed here is also much lower than those reported for nanostructures made of other materials as


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summarized in Refs. 3 and 4. By means of ion track template method, several FE structures have been reported, including gold nano-wires,9 copper nano-cones,15 copper nano-wires,38 structured gold nano-wire cathods,40 polycrystalline silver nano-wire arrays,41 and cobalt nano-wire arrays.42 These structures have shown the advantages of ion track method in field emission investigations. Some of them have shown a very high enhancement factors.15 All our three samples show similar maximum current densities of about 1.3 mA/cm2. Taking into account the fact that the areal density of nano-cones is only 8×105 cm-2 which is nearly three orders of magnitude smaller than that of cylindrical nano-wires (5×108 cm-2), the average emission current per nano-cone emitter is three orders of magnitude larger than that of a nano-wire emitter, illustrating significantly enhanced field emission properties of single nano-cone emitters compared to the shorter nano-wires. It is worthwhile pointing out that the emission current measured in our nano-cone sample is relatively stable over a whole measurement period of four hours at an intermediate vacuum of 1×10-5 Pa (see Figure S3 in the Supporting Information). Our long-term emission results show that the relative standard deviation (RSD) is 13% which is comparable to the reported 12% of Germanium nano-wire arrays7 and 10% of PrB6 Nanorods10. One should mention here that, according to previous studies, density of emitters influences the field emission via field shielding effects when the density is sufficiently large. One can imagine that larger density could induces stronger field shielding effects and this is planned in our followup work. Also note that the reproducibility of electrical characteristics is good since it is highly dependent on geometrical features.


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Figure 6. Field emission J-E curves for Cu nano-wires (a), and nano-cones with cone angle of 6.2° (b) and 2.9° (c), the insets are corresponding Fowler-Nordheim plots. The turn-on fields of nano-cones and nano-wires are shown in (d). The step-like field emission behavior in low applied fields appeared in Figure 6d are due to insufficient instrumental resolution during measuring emission current. For quantitative analysis, the Fowler-Nordheim (F-N) theory is usually employed to describe the field emission characteristics of materials. The current density J is expressed as36,37 3   − BEφ 2 β E exp J = AE φ  βE  2

2

    

(1)

or in a transformed form 3

A β2  B φ2 J  ln 2  = ln E  − E βE E   φ 

(2)


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where AE = 1.54 × 10-6 A eV V-2, BE = 6.83 × 103 eV-3/2µm-1, β is the field enhancement factor, E is the applied field, and ɸ is the work function of the emitter material. We can see from Eq. 2 that ln(J/E2) is linearly proportional to 1/E with a slope of -BEɸ3/2/β. In other words, the field enhancement factor β can be deduced from the slope of F-N plot. In our calculation, the work function ɸ for copper is taken as 4.46 eV.8 In Figures 6a-c, the insets are the corresponding F-N plots. The linear relationship of ln(J/E2) versus 1/E of each sample implies that the electron emission from Cu nano-wires and nano-cones follow the F-N behavior, i.e. a process of tunneling of electrons through a potential barrier. The calculated field enhancement factors are provided in the insets. The nano-cone sample with cone angle of 2.9° possesses the largest field enhancement factor of 6068 (1149 for the nano-cone sample with cone angle of 6.2° and 1041 for the nano-wire sample), thereby leading to the lowest turn-on field. Usually, the field emission enhancement factor can’t exceed a value over 2000 because in this case the emission tips should blow-out. For our nano-cone arrays with cone angle of 2.9°, the calculated enhancement factor is 6068 which is remarkably large and may be overestimated because of following two reasons. Firstly, nanomaterials, e.g. Cu nano-cones, should possess lower work function than the value of their bulk counterpart since the surface energy of nanomaterial is higher. In this case, the FE enhancement factor is generally overestimated. Secondly, field electron emission is enhanced by the emission of thermal electron at high temperature caused by the high current. In our case the thermal conductance of the 2.9° is expected to be lower than the 6.2° cones because of smaller cone angle, inducing a higher temperature. Thus as shown in Figure 6c, the FN plot for the cones of 2.9° is not a straight line and the slope becomes larger at high emission current region. Therefore this may also results in an overestimated enhancement factor.


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Table 1. Key parameters of field emission, geometry, structure, and material for various metallic nanostructure-based field emitters reported in previous literature and this work. Structure

Mat erial

Turn-on field @ 10 µA/cm2 [V/µm] 4.6 at emission starts

Field enhancement factor (β) 443

Alignment

Crystallinity

Effective height [µm]

Ref.

nano-wire

Cu

partly

twinned crystal

~10.5

[8]

Cu Cu

~4.3-10 12.4

380-800 713

vertically partly

single crystal

8-18 ~7

[38] [19]

nano-wire nano-wire nanopillar nano-wire nanothorn nano-wire

Ni W

4.0 6.2

1300 1578

randomly randomly

single crystal single crystal

-

[24] [39]

Au

4.0 at 1 nA

Max. 630

-

7-28

[40]

nano-wire

Ag

960

polycrystal

10

[41]

nano-wire nano-wire nano-wire whisker nano-cone nano-cone nano-cone nano-cone

Co Ge Mo W Cu Ni Ni Au

211 462 4400 1904 84-237 1500-2000 3720 2683

vertically vertically partly partly vertical vertically partly vertically

single crystal single crystal single crystal single crystal single crystal -

2.1 ~3.1 20 ~28 0.3-1.2 0.4 0.27

[42] [7] [43] [44] [15] [45] [30] [28]

nano-cone nanotaper nano-wire nano-cone (6.2°) nano-cone (2.9°)

AlN ZnO

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28

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The maximized field emission enhancement of the nano-cones with cone angle of 2.9° can be attributed to their large height, sharp tip, vertical alignment, intermediate cone angle, and singlecrystal structure. In principle, the field emission enhancement is related to the ratio of cone height and curvature radius at the apex of the emitters, i.e. the ratio of the nano-cone height and curvature radius at the apex.3,31 Apparently, a combination of large height and small radius would give rise to enhancement. In this work, the height of the nano-cones is 28 µm and the minimum curvature radius at the rounded tip, as shown in the inset of Figure 5a, is smaller than 3 nm. Thus, the highest ratio of cone height and curvature radius at the apex of nano-cones is


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larger than 9000, which generates huge field enhancement around the tip as demonstrated by the simulated electric field distribution (Figure 7a). Furthermore, for the flat tip shown in Figure 5b, there exists a strong field enhancement around the circular edge, as evidenced by the numerical simulation shown in Figure 7b. Both types of sharp features are beneficial to the local field enhancement. Secondly, vertical alignment should be another critical factor that induces pronounced field enhancement. It has been observed that random alignment of metal nanostructures diminishes the effective height of the emitters and induces field shielding over neighboring emitters. Thirdly, according to previous studies,2,47 the field emitters with larger cone angle have lower field enhancement. However, field emitters with larger cone angle have better thermal and mechanical stability. Therefore, an intermediate cone angle (2.9°) represents a balance between field emission enhancement and mechanical and thermal stability. Finally, a single-crystal structure that is free of grain boundaries is an additional advantage for field emission because scattering of electrons and phonons at the grain boundaries can significantly decrease the electrical and thermal conductivity of the material.

Figure 7. Simulated electric field profile for a single two-dimensional nano-cone with a rounded tip (a) or a flat tip (b). The white dashed line outlines the geometry of the nano-cone. (c) Simulated field strength versus tip diameter (black triangles for flat tip, red diamonds for rounded tip). The dashed lines are fits to simulations using formula (3).


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We have to point out that spacing is another geometric parameter which determines the strength of field screening effect. It has been reported that it is necessary to have individual vertically aligned emitters spaced apart by approximately twice their height in order to minimize field shielding effects.2 In this work the areal density f of the Cu nano-cones is approximately 8×105 cm-2 and the nano-cones have a Poisson distribution on the substrate due to the random hit of ions. The mean spacing, defined as 1 /( 2 f ) , is thus calculated to be 5.6 µm which is significantly smaller than twice height of the nano-cones. Such a small spacing is expected to generate strong field screening, however, a current density as large as 1.3 mA/cm2 has still be osberved in our experiments. This also indicates that it is possible to further increase the current density by increasing the separations between nano-cones in our future work. To further understand the enhanced field emission of nano-cones, finite element simulations have been performed to calculate the electrostatic field strength as a function of nano-cone tip shape and dimension by using the COMSOL Multiphysics software. The validation of the simulations was verified by a simple model of a “hemisphere on a plane” (details are presented in the Supporting Information). In the simulations, the distance between the anode and the nanocone tip is 300 µm and the voltage applied over the distance is 1500 V, corresponding to a macroscopic applied field of 5 V/µm. Figures 7a and 7b show the electric field profile for two nano-cones with (a) a rounded tip and (b) a flat tip. Their tip diameter, base diameter, and nanocone height are set to 6 nm, 1.47 µm, and 28 µm, respectively. Both structures show significant field enhancement around the tip region. The enhanced field tends to localize at the sharpest site (top of the rounded tip and edge of the flat tip), consistent with the field emission mechanism. The calculated field enhancement is lower than that of experimental observations, which is maybe due to inaccuracy of 2D simulations. It is known that numerical simulations based on the


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finite element method (FEM) have been widely adopted to understand the field enhancement of field electron emission. In our calculations, we use the identical geometrical parameters to those of our nano-cones which have very large height of 28 µm which is in strong contrast to their tip curvature radius of 3 nm. In such a case, two-dimensional (2D) simulations make calculations practicable. However, in 2D simulations, only two dimensions are considered whereas the length of the third dimension is deemed as infinite long. In such a condition, the 2D simulation does not yield an absolute value for the field enhancement and usually underestimate it, but it still allows us to reproduce the experimental tendency and qualitatively analyze the field distribution around the cone tip. To estimate the effect of tip diameter d, the field strength versus tip diameter is plotted in Figure 7c for both types of nano-cones. The field strength increases dramatically with decreasing tip diameter and the absolute field strength for the flat-tip nano-cone is slightly larger than the rounded tip. In addition, both curves can be perfectly fitted by Esim =

a db

(3)

where Esim is the simulated field strength, and a and b are size-dependent constants. This formula implies that the field strength can be further increased by sharpening the nano-cone tip. In the experiments, nanometer- and even subnanometer-level sharp tips could be achieved by posttreatment of the as-grown nano-cones using e.g. field-directed sputter etching.48 Therefore, a lower turn-on field and larger field enhancement factor would be expected, which we will investigate in follow-up work.

4. CONCLUSIONS In summary, we have shown that vertically aligned and mechanically self-supported singlecrystal Cu nano-cone arrays can be fabricated by electrochemical deposition in ion track


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templates with conical pores. The Cu nano-cones exhibit a maximal ratio of cone height and curvature radius at the apex up to 9000 and the cone angle can be adjusted between 0.3° and 6.2° by selecting suitable etchants to provide asymmetric etching of the ion track template. Benefiting from these unique geometrical and structural features, Cu nano-cones with a cone angle of 2.9° show dramatically enhanced field emission properties with a turn-on field as low as 1.9 V/µm and a field enhancement factor of up to 6068. The results suggest that the Cu nanocones fabricated by ion track technology are promising candidates for cold electron field emission and that the emission current (turn-on field) can be further enhanced (decreased) by increasing the separation of the nano-cones to surpress the field screening effect. Due to the flexibility of the fabrication strategy, the method also allows the generation of nano-cone structures based on other materials including metals (Au, Ag, Pt etc.) and semiconductors (e.g. CdS).

ASSOCIATED CONTENT

Supporting Information SEM micrographs of Cu nano-cone array, details of statistics of cone angles, emission lifetime of a copper nano-cone array, validation of the field emission simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors * Email: [email protected] (DYL) * Email: [email protected] (JL)


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Author Contributions ‖These authors contributed equally.

ACKNOWLEDGMENT The financial supports from the National Natural Science Foundation of China (Grant Nos.: 11175221, 11474240, 11375241, and 11179003) and the Hong Kong Research Grants Council (ECS Grant no. 509513), the Hong Kong Innovation and Technology Commission (ITF Grant no. ITS/196/13) and the West Light Foundation of the Chinese Academy of Sciences are acknowledged. JLD thanks Dr. Th. W. Cornelius, Dr. H. J. Yao, and Dr. D. Mo for their valuable discussions and Dr. J. T. Chen at LICP, CAS for the FE measurements.

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Table of Contents

Vertically-aligned single-crystal nano-cone array with controlled cone angle and enhanced field emission is reported. The Cu nano-cones were fabricated by ion track templates in combination with electrochemical deposition, and their cone angle is controlled in the range from 0.3o to 6.2o. These nano-cones show enhanced field emission with minimum turn-on electric field of 1.9 V/µm.


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Vertically-Aligned Single-Crystal Nanocone Arrays: Controlled

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