Field emission Properties of Tungsten Oxide Nanowires

Saqib, Muhammad (author), Remškar, Maja (mentor)

We studied field emission of electrons from two types of single crystalline tungsten oxide, W5O14 and W18O49 nanowires, which were synthesized by iodine transport method. The morphology, structure and work functions of these nanowires were investigated and compared. Raman spectrum of the W5O14 nanowires was measured for the first time. The work function of the W5O14 (4.29-4.34 eV) and W18O49 (4.55-5.57 eV) nanowires have been measured by Kelvin probe force microscopy in non-contact atomic force microscopy in ultra-high vacuum and with nanometre spatial resolution. The new methods were figured out in order to mount/fix the nanowires on different tip holders for field emission testing. Field emission measurements were carried out using individual W5O14 and W18O49 nanowires of 140-160 nm and 0.2-3 μm in diameter, respectively. The experiment were performed in two different, near field and far field regimes, in ultra-high vacuum chambers at a level of 1-3×10-9 mbar in different measurements setups. The obtained FE curves were analysed in the framework of the Fowler-Nordheim theory. Considering an average work function of 4.3 eV for W5O14, the field enhancement factors of about 110 ± 10, 180 ± 25 and 210 ± 30 were calculated at 2, 4 and 5 μm distances, respectively in the near-field regime. At macroscopic distances (⡥1 mm), the Fowler-Nordheim theory revealed unrealistically high field enhancement factors: for W5O14 at 1 mm it was 17000 ± 500, and for W18O49, the field enhancement factors were 5050 ± 30 and 6450 ± 30 at 600 μm and 800 μm emitter-collector distance, respectively. Therefore, another model proposed by R. Smith was discussed. Since the work function of a single nanoemitter has been measured, the field enhancement factors were obtained based on a realistic work function values. This is a key step towards quantifying the field emission properties of W5O14 nanowires with well-defined structure. The rigidness, low work function and high field enhancement factor of the W5O14 nanowires allow them to be considered as an effective candidate for electron sources in different applications, especially in field emission devices.

• Language: English
• Keywords: Tunsten oxides, Nanowires, Raman spectroscopy, Kelvin probe force microscopy, Work function, Field emission, Field enhancement factor
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-118128
• COBISS.SI-ID: 18439683
• Publication date in RUL: 21.08.2020

Secondary language

• Language: Slovenian
• Title: Lastnosti poljske emisije iz nanožičk volframovih oksidov

Abstract

Raziskali smo emisijo elektronov iz dveh vrst monokristalnih volframovih oksidov, W5O14 in W18O49 v obliki nanožičk, ki smo jih sintetizirali s transportno reakcijo z uporabo joda. Izmerili in analizirali smo obliko, strukturo in izstopno delo teh nanožic. Prvič smo izmerili Ramanski spekter nanožic W5O14. Izstopno delo nanožic W5O14 (4.29-4.34 eV) in W18O49 (4.55-5.57 eV) smo izmerili s Kelvinovo tipalno mikroskopijo v nekontaktni mikroskopiji na atomsko silo v ultra visokem vakuumu in z nanometrsko prostorsko ločljivostjo. Našli smo nove načine pritrjevanja nanožičk na različne nosilce za meritve poljske emisije. Meritve poljske emisije smo izvedli z uporabo individualnih nanožic W5O14 in W18O49 s premeri 140-160 nm oz. 0,2-3 μm. Testiranja so potekala v dveh različnih režimih, v bližnjem in daljnjem polju, v komorah z ultra visokim vakumom velikosti 1-3 × 10-9 mbar in pri različnih merilnih nastavitvah. Dobljene rezultate poljske emisije smo analizirali v okviru teorije Fowler-Nordheim. Ob upoštevanju povprečne vrednosti izstopnega dela 4,3 eV za W5O14, so bili faktorji ojačitve polja v režimu bližnjega polja približno 110 ± 10, 180 ± 25 in 210 ± 30 na razdaljah 2, 4 in 5 μm. Na makroskopskih razdaljah (⡥ 1 mm) je teoretični opis Fowler Nordheim odkril nerealno visoke faktorje ojačitve polja: za W5O14 pri oddaljenosti 1 mm med koncem žičke in kolektorjem elektronov je bilo ojačenje 17000 ± 500, za W18O49 pa 5050 ± 30 in 6450 ± 30 pri razdaljah 600 μm in 800 μm. Zato smo uporabili drugačen model, ki ga je predlagal R. Smith. Faktorje ojačitve polja smo izračunali na podlagi izmerjenega izstopnega dela na posameznih nanožičkah, kar predstavlja ključno izboljšavo pri kvantifikaciji lastnosti emisij elektronov iz nanožičk W5O14 z dobro opredeljeno strukturo. Emisijske lastnosti nanožičk W5O14, kot so njihova togost, nizko izstopno delo in visok faktor ojačitve polja omogočajo, da jih predlagamo kot kandidate za učinkovite vire elektronov za raznovrstno uporabo, predvsem v napravah na principu poljske emisije.

• Keywords: Volframovi oksidi, Nanožičke, Ramanska spektroskopija, Kelvinova tipalna mikroskopija na atomsko silo, Izstopno delo, Poljska emisija, Faktor ojačitve polja