Characterization of MgO and NaCl as Decoupling Layers and Investigation of the Structural and Electronic Properties of C60

Ebenau, Lucas Kjær ; Ahmed, Metin

4. term (FYS10)

Physics, Master

2024

2024-05-31

46

Metaloverflader kan i høj grad påvirke, hvordan molekyler opfører sig. For at undersøge molekyler mere på deres egne præmisser brugte vi ultratynde isolerende film til at afkoble dem elektronisk fra et sølvsubstrat. Vi voksede MgO på Ag(100) og NaCl på Ag(111) som ordnede (epitaksiale) tyndfilm ved hjælp af en Knudsen-celle i ultrahøjt vakuum og undersøgte prøverne med et lavtemperatur scannende tunnelmikroskop (STM). Selvom begge materialer tidligere var blevet brugt i vores STM-opsætning, justerede vi vækstbetingelserne for at opnå større dækning af adsorberede C60-molekyler. For at forhindre, at molekylerne diffunderede væk, måtte prøverne køles ned til mindst −25 °C. Vi karakteriserede båndgabet for begge isolerende film og udførte IPS-målinger for at vurdere overfladens elektroniske egenskaber for de anvendte krystaller, isolatorer og molekyler. Ud fra disse data konkluderer vi, at både MgO og NaCl effektivt afkobler molekylerne fra sølvsubstratet. Endelig målte vi C60’s grænseorbitaler—HOMO (højest besatte), LUMO (lavest ubesatte) og LUMO+1—og analyserede den observerede spaltning og bredning af LUMO+1 samt forholdet til HOMO–LUMO-gabet.

Metal surfaces can strongly influence how molecules behave. To study molecules more on their own terms, we used ultrathin insulating films to electronically decouple them from a silver substrate. We grew MgO on Ag(100) and NaCl on Ag(111) as ordered (epitaxial) thin films using a Knudsen cell in ultra-high vacuum, and examined the samples with a low-temperature scanning tunneling microscope (STM). Although both materials had been used in our STM setup before, we adjusted the growth conditions to achieve higher coverage of adsorbed C60 molecules. To keep the molecules from diffusing away, the samples had to be cooled to at least −25 °C. We characterized the band gap of both insulating films and performed IPS measurements to assess surface electronic properties for the crystals, insulators, and molecules used. From these data, we conclude that both MgO and NaCl effectively decouple the molecules from the silver substrate. Finally, we measured the C60 frontier orbitals—HOMO (highest occupied), LUMO (lowest unoccupied), and LUMO+1—and analyzed the observed splitting and broadening of LUMO+1, discussing how these features relate to the HOMO–LUMO gap.

[This summary has been rewritten with the help of AI based on the project's original abstract]

Isolerende ; NaCl ; MgO ; C60 ; STS ; STM ; HOMO ; LUMO ; IPS ; Båndgab ; Overfladefysik ; Overfladetisltande ; Epitaxy

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