Karakterizacija konic za vrstični tunelski mikroskop z vrstičnim elektronskim mikroskopom

Anžlovar, Jan (author), Kladnik, Gregor (mentor), Midden, Marion van (comentor)

Pri izbiri primernih konic za vrstični tunelski mikroskop (VTM) je zelo pomembno, da lahko ustrezno določimo velikost in obliko konice, saj morajo biti prave dolžine in ostrine, hkrati pa ne smejo biti umazane ali poškodovane. To je samo z optičnim mikroskopom težko določiti, saj zaradi valovne dolžine svetlobe ločljivost ni primerna, da bi lahko konice dovolj natančno karakterizirali. Zato se pri tem namesto optičnega uporablja vrstični elektronski mikroskop (VEM), ki za svoje delovanje namesto svetlobe uporablja curek elektronov. Elektrone v VEM-u pospešimo s potencialom več kilovoltov, zaradi česar je njihova valovna dolžina manjša od valovne dolžine svetlobe in zato omogoča slikanje vzorcev z bistveno večjo ločljivostjo. Konice za VTM se pripravljajo tako, da se v raztopini natrijevega hidroksida (NaOH) pod napetostjo pojedka volframovo žičko, s čimer se ta na koncu ušpiči. Na obliko konice vpliva veliko faktorjev, zato moramo biti pozorni, pri kakšnih pogojih jedkamo. Pri opazovanju z elektronskim mikroskopom curek elektronov usmerjamo v opazovane konice, nato pa zajemamo različne signale z uporabo specializiranih detektorjev. Elektroni iz curka interagirajo z atomi v vzorcu (konici), kjer pride do sipanja elektronov iz atomov, ki jih zaznamo z detektorjem sekundarnih elektronov (DSE), ali pa do odboja primarnih elektronov pri elastičnih trkih z jedrom, ki jih lahko zajamemo z detektorjem povratno sipanih elektronov (DPS). Namen zaključne naloge je bil pripraviti konice za VTM z metodo jedkanja, se seznaniti z delovanjem VEM-a ter ga uporabiti za slikanje konic. Zajete slike je bilo nato potrebno obdelati ter izmeriti pomembne karakteristike konic, kot sta npr. njihova dolžina ter ostrina, ter končno presoditi, katere so primerne za nadaljnjo uporabo z VTM. Na podlagi ugotovitev, pri katerih pogojih nastanejo primerne konice, je končni cilj naloge podati ustrezne parametre izdelave konic ter na kaj je potrebno biti pozoren, da se konice ne uničijo ali umažejo.

• Language: Slovenian
• Keywords: Vrstični tunelski mikroskop, karakterizacija konic, optični mikroskop, vrstični elektronski mikroskop, elektrokemijsko jedkanje.
• Work type: Final paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2025
• PID: 20.500.12556/RUL-173705
• COBISS.SI-ID: 251473411
• Publication date in RUL: 20.09.2025

Secondary language

• Language: English
• Title: Characterization of tips for scanning tunneling microscopy using a scanning electron microscope

Abstract

When selecting suitable tips for a Scanning Tunneling Microscope (STM), it is very important that we can correctly determine the size and the shape of the tip for they must be of appropriate length and sharpness while they must not be contaminated or damaged. This is hard to determine only with optical microscope because the resolution is not appropriate to characterize tips precisely enough because of the wavelength of light. Therefore, instead of optical Scanning Electron Microscope (SEM) is used which uses electron beam instead of light for its functioning. Electrons in SEM are accelerated with potential of more kilovolts for which their wavelength is smaller than wavelength of light therefore they enable us to take photos of the sample with significantly higher resolution. STM tips are prepared by electrochemically etching a tungsten wire in a NaOH solution, which sharpens the wire at its end. The shape of the tip depends on many factors therefore we have to be really careful to the conditions of the etching. While imaging with electron microscope we direct electron beam in observed tips and then detect different signals using specialized detectors. Electrons from the beam interact with atoms in the sample (tip) where electrons are scattered out of the atoms and we can see them using Secondary Electron Detector (SED) or it comes to backscattering of the primary electrons through elastic collisions with the nucleus which we can detect with Backscattered Electron Detector (BSD). The aim of this thesis was to prepare tips for STM with the etching method, get familiar with operating SEM and use it to image tips. Captured images were needed to be processed and we had to measure important characteristics of the tips like for instance their length and sharpness and finally decide which ones are appropriate for further use in STM. Based on the findings at which conditions appropriate tips are made the final goal is to give relevant parameters of tip production and what we have to look for so tips do not get damaged or contaminated.

• Keywords: Scanning tunneling microscope, tip characterization, optical microscope, scanning electron microscope, electrochemical etching.