1© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com

Enhanced Field Emission of WS 2 Nanotubes

G. Viskadouros , A. Zak , M. Stylianakis , E. Kymakis , R. Tenne , and E. Stratakis *

emitters showing low turn-on voltage, high current emis-

sivity and increased durability under poor vacuum condi-

tions. Field emission (FE) is of great commercial interest in

vacuum micro/nanoelectronic devices, such as electron guns

and microwave power amplifi ers, and everyday electronic

devices, such as fl at panel FE displays (FEDs). [ 1 ] Owing to

their unique geometries, 1D nanostructures like NTs, nano-

fi bers and nanohorns have attracted signifi cant interest for

their potential FE applications. Indeed, due to their small

curvatures (few nanometers) and relatively long lengths (few

microns), these high aspect ratio nanostructures can generate

a large electric fi eld enhancement capable of obtaining elec-

tron emission at low applied electric fi elds.

Among the numerous 1D nanostructures tested carbon

nanotubes (CNTs) were found to be the best fi eld emit-

ters. [ 2–4 ] Even though Toshiba, Canon and Samsung are

known to be preparing to commercialize FEDs based on

carbon nanotubes (CNTs), [ 5 ] there are no reliable con-

sumer production models available yet. One of the main

reasons is the lack of long-term emission stability, [ 6 ] attrib-

uted to the fast degradation rate of CNTs, regardless of

their high electron emission density. [ 7 ] Later on, research in

the fi eld was focused on the development of FE cathodes

using 2D layered materials, like graphene. [ 8,9 ] Recently,

great technological interest has been placed on 2D single

layer semiconducting materials, which are the inorganic

analogues of graphene, exhibiting unique physical, optical,

and electrical properties correlated with its 2D ultrathin

atomic layer structure. [ 10,11 ] However, to date, there is only

one study on the FE properties from such materials, in par-

ticular MoS 2 sheets. [ 12 ] Besides this, there is no report on

the FE performance of NTs made of inorganic layer struc-

tured materials.

Results on electron fi eld emission from free standing tungsten disulfi de (WS 2 ) nanotubes (NTs) are presented. Experiments show that the NTs protruding on top of microstructures are effi cient cold emitters with turn-on fi elds as low as 1 V/µm and fi eld enhancement of few thousands. Furthermore, the emission current shows remarkable stability over more than eighteen hours of continuous operation. Such performance and long-term stability of the WS 2 cathodes is comparable to that reported for optimized carbon nanotube (CNTs) based emitters. Besides this, it is found that the WS 2 cathodes prepared are less sensitive than CNTs in chemical reactive ambients. The high fi eld enhancement and superior reliability achieved indicates a potential for vacuum nanoelectronics and fl at panel display applications.

Nanoelectronics

DOI: 10.1002/smll.201303340

Mr. G. Viskadouros, Dr. E. Stratakis Institute of Electronic Structure and Laser (IESL) Foundation for Research and Technology-Hellas (FORTH) Heraklion 71110 , Crete , Greece E-mail: stratak@iesl.forth.gr

Dr. E. Stratakis Department of Materials Science and Technology University of Crete 71003 , Heraklion , Crete , Greece

Mr. G. Viskadouros Technical University of Crete 73100 , Hania , Greece

Dr. A. Zak Department of Science Holon Institute of Technology P.O. Box 305 , Holon 58102 , Israel

Mr. G. Viskadouros, Mr. M. Stylianakis Prof. E. Kymakis Electrical Engineering Department and Center of Materials Technology & Laser School of Applied Technology Technological Educational Institute of Crete Heraklion 71004 , Crete , Greece

Prof. R. Tenne Department of Materials and Interfaces Weizmann Institute Rehovot 76100 , Israel

1. Introduction

In the last two decades intensive research effort has been

devoted to the design and fabrication of fi eld electron

small 2014, DOI: 10.1002/smll.201303340

G. Viskadouros et al.

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full papers

In this manuscript the FE properties of WS 2 NTs are

explored. WS 2 is a layered structure comprising 2D sheets of

S-W-S atomic trilayer units, while the sheets are loosely bonded

via van der Waals forces. Following the discovery of WS 2 NTs

in 1992 [ 13 ] this nanomaterial became available in large quanti-

ties [ 14 ] and it has been extensively studied. The initial results on

the optical and electrical properties of the individual nanotubes

were recently reported. [ 15–17 ] It was shown that due to their rela-

tively small bandgap, small amount of dangling bonds, long term

mechanical stability and nontoxicity, WS 2 NTs are potentially

promising for diverse technological applications. [ 18 ] We found

that WS 2 NTs also exhibit remarkable FE performance and sta-

bility comparable to carbon nanostructure-based fi eld emitters.

2. Results and Discussion

Figure 1 a and b show representative transmission electron

microscopy (TEM) images of the as-prepared WS 2 NTs. As

estimated from TEM analysis the length of the WS 2 NTs varied

between 2–20 microns while the diameter was 30–120 nm. The

radial direction, perpendicular to the growth axis of the nano-

tube, is the c-axis (002), while the interlayer distance is 0.63 nm.

For fi eld emission studies, the as prepared NTs were dis-

persed into a semiconductive poly(3-hexylthiothene) (P3HT)

matrix, dissolved in acetone (20% WS 2 NTs/P3HT solution)

and further solution-casted on fl at Si (FSi) as well as on low

and high-aspect ratio Si microspike arrays (LµSi and HµSi

respectively) fabricated by ultrafast laser processing. Samples

of the pristine NTs deposited on planar and microstructured

substrates without using a polymer matrix have been also pre-

pared and used as controls. Micro-Raman spectroscopy per-

formed on NTs/µSi and reference NTs/FSi samples revealed

typical spectra (Figure S1) of the WS 2 NTs [ 19 ] indicating that

the structure of the NTs is preserved through the processes.

As shown in Figure 2 a the Si microspikes are well-

separated and are perpendicular to the substrate surface.

small 2014, DOI: 10.1002/smll.201303340

Figure 1. Representative TEM images of bundles of WS 2 NTs, (a) and a single one (b). The radial direction, perpendicular to the growth axis of the nanotube, is the c-axis (002), while the interlayer distance is 0.63 nm.

Figure 2. Scanning electron microscopy (SEM) view (45°) of: (a) A laser etched high-aspect ratio Si microspike array, HµSi, (2.25 J/cm 2 , 500 pulses at 1 kHz, 500 Torr SF 6 ), (b) WS 2 NTs/HµSi emitters formed by drop casting 10 µl of 10% WS 2 NTs/P3HT solution on the Si spikes shown in (a); (c) WS 2 NTs/LµSi emitters formed by drop casting 10 µl of 10% WS 2 NTs/P3HT solution on the low-aspect ratio microspike array, LµSi, (0.35 J/cm 2 , 500 pulses at 1 kHz, 500 Torr SF 6 ); (d) High magnifi cation images of free standing WS 2 NTs.

Enhanced Field Emission of WS 2 Nanotubes

3www.small-journal.com© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

After drop-casting the WS 2 NTs/P3HT solution, the micro-

spikes were partially (HµSi-Figure 2 b) or completely (LµSi-

Figure 2 c) covered by the nanocomposite, while some NTs

protruded out of the P3HT matrix and decorated the top of

the micro-spikes, forming a random array of free-standing

NTs (Figure 2 d). On the contrary, NTs were observed to

aggregate, while few of them could protrude, for the sam-

ples prepared without using the polymer matrix (Figure S2).

It can be concluded that the microspike substrate comple-

mented with the polymer-NT composite is a suitable system

to exploit the high aspect ratio of WS 2 NTs for different

applications. In the present case, it allowed direct comparison

of the FE performance of WS 2 NTs that lay fl at on a planar

substrate with that of free standing ones.

The high aspect ratio of the free-standing NTs is an

ideal property for fi eld emission. Initially, the emission per-

formace of NTs with and without using a polymer matrix

is compared. It is observed that the performance of the

NTs-polymer composite cathodes were always superior. In

particular, the WS 2 NTs-P3HT cathodes on planar Si per-

formed better than the WS 2 NTs ones. The difference can

be attributed to the NTs preferential orientation at dif-

ferent angles relative to the planar substrate as a result

of the effect of the polymer, which forces the nanotubes

to protrude perpendicualr to the substrate (Figure S3). [ 20 ]

Aggregation of the NTs may also be a reason for the infe-

rior performance of cathodes without polymer. Besides

this, the use of a polymer matrix was necessary to observe

protruding NTs onto microstructured substrates. This inter-

esting formation of free-standing NTs can be attributed to

the low surface tension of NTs/P3HT solution and the large

substrate roughness. [ 8 ] Alternatively, the FE performance

improvement can be attributed to the fact that the con-

ducting polymer composite serves as an intermediate layer

between the NT emitters and a conducting substrate, which

improves the electrical contact i.e. reduce the contact resist-

ance, between the two. [ 21 ]

Owing to its superiority only the FE performance results

of the NTs-polymer cathodes are presented in the following.

In all cases, the FE performance and stability of as-prepared

bare µSi substrates were initially tested and compared to

those obtained after the NTs deposition (NTs/µSi cathodes).

It is always found that the performance as well as the dura-

bility of NTs/µSi cathodes was by far superior (Figure 4S

and 5S). Figure 3 a shows the current density-electric fi eld

(J-E) characteristics of the NTs/µSi and NTs/FSi cathodes

(NTs means NTs-polymer). The turn-on fi eld, E to , which is

defi ned as the macroscopic fi eld at which the emitted current

exceeds the background noise current was as low as ∼1.1 V/

µm for the NTs/HµSi cathode. While, the threshold fi eld, E th is defi ned as the macroscopic fi eld where the emission cur-

rent density, J , becomes ∼1 µA/cm 2 , was as low as ∼1.5 V/µm

for the NTs/HµSi. This value is almost three times lower from

that measured on the best NTs sample on planar Si. Further-

more it is comparable to that of vertically oriented CNTs [ 22 ]

(∼1.0 V/µm, at 1 µA/cm 2 ), WO x nanorods [ 23 ] (∼1.2 V/µm at

1 µA/cm 2 ), carbon nanowalls [ 24 ] (∼1.5 V/µm at 1µA/cm 2 ),

MoS 2 sheets [ 12 ] (∼ 2.7 V/µm at 1 µA/cm 2 ) and free standing

graphene sheets [ 14 ] (∼ 2.0 V/µm at 1 µA/cm 2 ).

Above the threshold fi eld, the FE current increased

exponentially in accordance with Fowler-Nordheim (FN)

tunneling model, until saturation was reached. A similar

saturation effect was observed in CNT fi lms and attrib-

uted to adsorbents on the emitter tip [ 25 ] and a large voltage

drop along the emitter and/or at the emitter/substrate inter-

face. [ 26,27 ] Accordingly, a high WS 2 /Si contact resistance may

lead to ineffi cient electron supply at the maximum attainable

current from the conduction band of Si that limits the emis-

sion current at high fi elds. The latter can be improved upon

using microspike arrays patterned on a heavily doped, thus

highly conductive n + -Si substrate (0.003 Ohm·cm. instead of

2 Ohm·cm), as presented in Figure 3 a (open circles). Indeed

it is observed that the maximum current density can be

increased by almost two order of magnitude, without prac-

tically any change of the FE properties at low fi elds upon

depositing WS 2 NT s on a highly conducting Si substrate. For

the study of the FE data we adopted the FN analysis [ 28 ] of

fi eld-assisted tunneling, which is widely used to describe the

relationship between the current density and the local fi eld at

the emitter E loc , which is usually related to the average fi eld

E as follows:

small 2014, DOI: 10.1002/smll.201303340

Figure 3. (a) Plot of J-E FE characteristics for the different WS 2 NTs samples: on Flat Si (black spheres); on low-aspect ratio microspike array, LµSi (blue spheres); on high-aspect ratio microspike array, HµSi (red spheres); The open circles correspond to the J-E characteristic of WS 2 NTs on an HµSi array fabricated using a heavily doped n + -Si wafer (HµSi + ). (b) The Fowler-Nordheim [ln(J/E 2 ) vs E −1 ] plots corresponding to J-E curves of (a). The solid lines represent linear fi ts to the FN equation.

G. Viskadouros et al.

4 www.small-journal.com © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

full papers

E E V

dloc β β= =

(1)

where V is the voltage bias, d is the inter-electrode spacing

and β is the geometric fi eld enhancement factor. Within this

frame the FN law is expressed as

exp2J A E b

EFNβ β( )= −⎛

⎝⎜⎞⎠⎟

(2)

where A = 1.54 × 10 −6 A eV V −1 is a constant which depends

on the surface structure, [ 28 ] b FN = 0.94 BΦ 3/2 with B =

6.83 × 10 7 V cm −1 eV −3/2 , while Φ is the work function of the

material in eV. Figure 3 b shows FN plots of the FE data for

the samples measured. The corresponding fi eld enhancement

factors (also shown in Figure 3 a) are determined by fi tting

the linear part of the data, following Equation ( 2) , assuming

a typical work function for WS 2 of 5.1 eV. [ 29 ] The corre-

sponding E th and β values suggest that the lower the emission

threshold the higher the enhancement factor obtained. If the

nature of emitting sites is the same in the samples measured,

then according to equation E loc = βE , the local fi eld at the ini-

tiation of the electron emission, βE to , should be constant. As

can be extracted from Figure 3 and Table S1, βE to is indeed

found similar for WS 2 -based samples, including that on fl at

Si, indicating that the emission sites are of the same nature in

all WS 2 -based cathodes.

Besides the geometric factor (aspect ratio), lifetime and

stability of the FE current over time are equally important

factors for choosing a cathode material for applications.

Figure 4 a presents the evolution of the emission current den-

sity at constant bias voltage over a long period of continuous

operation for the best emitting sample NTs/µSi. Results are

presented for three different vacuum levels, namely 10 −6 , 10 −5

and 10 −4 mbar. It is interesting that in the former case the

current density gradually increases until it becomes stable,

while current fl uctuations were observed to be within ±10%

of average value. Under these conditions, no degradation of

the emission performance was observed, even after eighteen

hours of continuous operation. A probable cause for the

emission improvement is consistent with a decrease in the

emitter’s resistance. Electrical conductivity measurements

performed under controlled enviromental conditions showed

that water molecules absorbed on the surface of WS 2 NTs

cause a decrease in the carrier mobility and conductivity of

WS 2 NTs. [ 30 ] Accordingly, the high electron densities emitted

during the FE process could give rise to joule heating of the

NTs apex and consequently to removal of the adsorbed water

molecules. On the other hand, the emission performance

gradually decreases upon exposure to 10 −5 Torr of O 2 , while

it is rapidly suppresed at higher oxygen levels. Furthermore

current fl uctuations become more pronounced. The degrada-

tion of the emission characteristics is reported to be due to

the combination of several phenomena, such as bombard-

ment from gas molecules ionized by the emitted electrons, [ 31 ]

chemical interaction with ambient gas molecules [ 32 ] and resis-

tive (joule) heating. [ 33,34 ] Besides this, current fl uctuations

may be attributed to adsorption/desorption of residual gas

molecules. [ 31 ] Depending on the nature of adsorbates (elec-

tropositive/electronegative) the local work function of the

emitter is increased or decreased giving rise to random emis-

sion current variations.

To distinguish between current degradation due to ion

bombardment and due to surface chemical interactions, we

compared the FE performance observed upon O 2 exposure

with that upon N 2 . As shown in Figure 4 b operation of NTs

under 10 −5 Torr of N 2 shows little or no effect on the FE cur-

rent stability. The gradual increase in current fl uctuations is

consistent with sputter cleaning of adsorbates by N 2 . This

behavior is different from that observed for operation under

10 −5 Torr of O 2 , where the FE current slowly decreases. Most

of the initial current, however, can be recovered by removing

the oxygen from the test chamber and allowing the emitter to

operate in 10 −6 mbar for many hours. Such reversibility indi-

cates that the curent decrease may be attributed oxygen attach-

ment on the NTs surface by weak van der Waals interactions,

though not from permanent structural damage. [ 35 ] The attached

oxygen molecules could be subsequently desorbed during

the NTs operation in high vacuum again. This phenomenon

small 2014, DOI: 10.1002/smll.201303340

0 2 4 6 8 10 12 14 16 18

10-8

10-7

10-6

10-5

10-4

10-3

P~10-6 mbar

P~10-5 mbar, O2

P~10-4 mbar, O2J

(A/c

m2 )

Operation time (h)

(a)

0 2 4 6 8 10 12 14 16 1810-7

10-6

10-5

10-4

10-3

WS2 NTs P~10-5 mbar, N

2

WS2 NTs P~10-5 mbar, O

2

CNTs P~10-5 mbar, O2

J (A

/cm

2 )

Operation time (h)

(b)

Figure 4. (a) Emission current stability over time at a constant bias voltage for WS 2 NTs/HµSi; results on three different O 2 pressures are presented. (b) Emission current stability over time at a constant bias voltage for WS 2 NTs/HµSi for 10 −5 mbar N 2 and O 2 environments. The results for the O 2 ambient are compared with those of optimized CNT fi eld emission cathodes.

Enhanced Field Emission of WS 2 Nanotubes

5www.small-journal.com© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

is probably analogous to metallic emitters during fi eld des-

orption cleaning. [ 36 ] On the contrary exposure to a higher O 2

pressure of 10 −4 Torr leads to substantial irreversible damage

and subsequent operation in high vacuum could not result in

complete recovery of the emission current. Since the equiva-

lent exposure to N 2 leads to little degradation, the irreversible

decrease involves a chemically enhanced deterioration, i.e. oxi-

dation of the WS 2 NT tip to e.g. WO x . Resistive heating of the

NTs apex [ 34,35 ] above 250 °C could be the reason for such oxi-

dation. [ 35 ] This is possible, considering that heating of different

types of semiconducting NWs and NTs from ∼500 to ∼1300 °C

had been measured and predicted for similar current densities

measured here. [ 34,37,38 ] However, one has to measure and/or

estimate the exact temperature rise to clarify this issue.

Ambient insensitivity is a key property of a FE cathode,

necessary to realize the lifetimes required for applications.

For instance, CNTs exhibit a permanent decrease in the FE

current density and an F th increase in oxygen rich environ-

ments. [ 32 ] This degradation phenomenon greatly impacts

CNTs application, especially in FEDs, under poor vacuum or

gas fi lled environments. To evaluate the degree of FE dete-

rioration upon exposure to oxygen in this case, we have com-

pared the lifetime of WS 2 NTs/HµSi cathodes with that of

planar SWCNT mats [ 4 ] at the same O 2 pressure. The results

are presented in Figure 4 b. It is clear that the WS 2 NTs/HµSi

cathodes exhibit a much longer lifetime than that of planar

SWCNT mats. Such behavior may be reasonable considering

that WO x nanostructures are reported to be good fi eld emit-

ters as well. Alternatively it may be attributed to the semi-

conducting properties of the WS 2 NTs. [ 39 ] It should be noted

that the type of NTs measured is different in each case, i.e.

WS 2 NTs were multi-walled while CNTs were single-walled.

Experiments are currently in progress to compare different

types of CNTs with WS 2 ones and clarify this issue.

3. Conclusions

In conclusion, the FE performance, stability and lifetime of

free-standing WS 2 NTs have been demonstrated. When a

solution of WS 2 NTs/P3HT is deposited on Si micro-spikes,

WS 2 NTs are anchored on the vertices of the spikes and

oriented nearly perpendicular to the substrate surface. It is

shown that this methodology leads to WS 2 NTs-based fi eld

emitters with comparable threshold fi eld and superior sta-

bility compared to CNT mats. The high fi eld enhancement

and long-term stability achieved indicates that the WS 2 NTs

are promissing candidates for vacuum nanoelectronics and

fl at panel display applications.

4. Experimental Section

Synthesis of WS 2 NTS : WS 2 NTs were synthesized using the high temperature solid-gas synthetic approach based on the reaction of H 2 S and H 2 gases with precursor tungsten oxide nanoparticles. [ 14 ] A new design of the fl uidized bed reactor was employed in the production of the WS 2 NTs. In this process a

sequence of the one pot reactions leads to the NTs preparation. In the fi rst step loosely agglomerated oxide nanospheres of less than 100 nm in diameter are converted into the oxide nano-whiskers of up to 50 µm in length. This reaction became possible due to the mild reduction of the precursor oxide into the inter-mediate volatile tungsten suboxide phase served as a building material for the oxide nanowhiskers growth. Subsequently, the oxide nanowhiskers are converted into hollow multiwall tung-sten disulfi de NTs by an outside inwards oxygen/sulfur exchange reaction.

Purifi cation of WS 2 NTs : WS 2 NTs were prepared according to the purifi cation process as described below. Briefl y, raw WS 2 NTs (0.5 g) were dispersed in acetone (250 mL) by ultrasonica-tion for 30 min, at RT, to give a homogenous suspension. After the completion of the ultrasonic bath treatment, the dispersion was left undisturbed for 5–10 min and the supernatant was decanted carefully, leaving the sediment in the bottom. The supernatant was fi ltered (0.45 µm, PTFE), gathering WS 2 NTs on the fi lter membrane. The fi lter was dried at 60 °C for 10 min, in order to easily isolate the WS 2 NTs powder (0.4 g, yield 80%) from it.

Preparation of WS 2 NTs/P3HT composites : The polymer P3HT (5 mg) was dissolved in THF (1 mL). A few drops of the solution were mixed with loose WS 2 NTs powder (1 mg), by hand, until a homogeneous viscose paste will be prepared. Finally, the rest of the dissolved polymer was added gradually and the obtained homogeneous mixture (20% ratio to the polymer) was magneti-cally stirred for 20 min. Thereafter, the mixture was sonicated for 10 min and fi nally was left undisturbed for 15 min, in order to settle down any agglomerates. A few drops from the supernatant were dribbled on a µSi substrate.

Preparation of μSi substrates : Microstructuring of the fl at sil-icon (Si) substrate surfaces was performed using ultrafast laser irradiation of single crystal n-type Si(100) wafers under reactive gas (SF 6 ) atmosphere. The irradiating source was a regenerative amplifi ed Ti:Sapphire laser (λ ∼ 800 nm) delivering 180 femto-second pulses at a repetition rate of 1 kHz. The laser pulse fl uence, set to 0.35 J/cm 2 for LµSi and 2.47 J/cm 2 for HµSi respectively, infl uences the surface density and the aspect ratio of the spikes on the textured surface. Areas of 10 × 10 mm 2 were patterned via scanning the laser beam onto the wafer surface. More details can be found elsewhere. [ 40 ]

Field emission measurements : Field emission measurements were performed under high vacuum conditions (< 10 −6 Torr), using the samples as cold cathode emitters in a short-circuit protected, planar diode system. Current – voltage ( I-U ) curves were taken at 200 µm anode-cathode distance, controlled by a stepper motor. Several emission cycles were taken in order to verify the stability and the reproducibility of the I-U curves. A voltage with variable sweep step, supplied by a HV source (PS350 – Stanford Research Systems), was applied between the anode and the cathode to extract electrons. The emission current was measured using an electrometer (Keithley 450) protected against high voltage surges by a MOSFET limiter. The stability of the emission current over time was examined by monitoring the evolution of the emitted current density over a long time period of continuous operation. Ultrahigh purity grade ∼99.999% N 2 , and O 2 gases were introduced through leak valves into the chamber.

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small 2014, DOI: 10.1002/smll.201303340

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the Integrated Initiative of European Laser Research Infrastructures LASERLAB-III (Grant Agreement No. 2012-284464). The authors acknowledge Ms. A. Manousaki for her support with the Field Emission Scanning Electron Micro-scope. We acknowledges the support of the EU COST action MP0902 (COINAPO). R.T. is the director of Helen and Martin Kimmel Center for Nanoscale Science and holds the Drake Family Chair in Nanotechnology.

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