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6CCVD Research

Control over Structure Formation of Small Molecular Weight Thiophenes in Vacuum Deposited Films

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-09-03
JournalAdvanced Materials Interfaces
AuthorsMatti Knaapila, Mathias K. Huss‐Hansen, Jakob Kjelstrup‐Hansen
InstitutionsUniversity of Southern Denmark, Norwegian University of Science and Technology
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This review surveys the structural control and polymorphism of small molecular weight thiophenes, focusing on Naphthyl-end-capped derivatives (NaT2, NaT3) deposited via vacuum methods.

  • Model System Validation: Naphthyl-terminated thiophenes (NaT2, NaT3) are established as highly stable and crystalline model organic semiconductors, enabling high-quality X-ray diffraction signals even from monolayers.
  • Substrate-Driven Polymorphism: Film structure, including unit cell parameters, strain fields, and molecular orientation (edge-on/standing-up vs. face-on/lying-down), is precisely controlled by the choice of substrate (Si/SiO2, OTS, Graphene, MoS2, Mica).
  • In Situ Growth Monitoring: Grazing-incidence X-ray scattering (GIWAXS/GISAXS) tracks the growth mode, revealing a transition from a frustrated 2D wetting layer to a 3D bulk crystal structure (Stranski-Krastanov type growth).
  • Structural Stability: NaT2 films maintain exceptional lattice parameter stability (less than 1% change) during prolonged in operando Organic Field-Effect Transistor (OFET) cycling (0 to -40 V gate voltage).
  • High-Pressure Phase Transition: Single-crystal NaT2 undergoes a sharp, reversible second-order phase transition at approximately 3.5 GPa, accompanied by bandgap closure (color change from yellow to red) and the formation of new sulfur-hydrogen (S…H) intermolecular interactions.
  • Interface Engineering: Functionalizing substrates with Octadecyltrichlorosilane (OTS) or using 2D materials (MoS2, Graphene) allows for targeted control over grain morphology (terraces vs. needle-like fibers) and molecular alignment.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
NaT2 Bulk Unit Cell (a)20.55AngstromMonoclinic, P21 symmetry (Ambient conditions)
NaT2 Bulk Unit Cell (b)5.96AngstromMonoclinic, P21 symmetry (Ambient conditions)
NaT2 Bulk Unit Cell (c)8.12AngstromMonoclinic, P21 symmetry (Ambient conditions)
NaT2 High-Pressure Transitionapprox. 3.5GPaSecond-order phase transition point (reversible)
NaT2 Lattice Stability (OFET)less than 1%Change in lattice parameters during 0 to -40 V gate cycling
NaT2 Molecular Layer Thicknessapprox. 2.1nmThickness of a single standing-up (edge-on) molecular layer
Alpha-6T HT Phase Transitionapprox. 290°CSubstrate temperature for transition to High-Temperature (HT) phase
Alpha-3T Unit Cell (a)15.35AngstromMonoclinic, P21/c symmetry (Unsubstituted prototype)
Film Thickness Range Studied2 to 55nmRange analyzed using GIWAXS/GISAXS

Key Methodologies

Section titled “Key Methodologies”
  1. Molecular Beam Epitaxy (MBE) / Vacuum Deposition: Used for the controlled growth of ultra-thin films (from monolayers up to tens of nanometers) of small molecular weight thiophenes (NaT2, NaT3, NCOH, alpha-6T).
  2. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS): The primary technique for determining unit cell structure, molecular orientation (edge-on vs. face-on), and crystalline order in thin films, often performed in situ during deposition.
  3. Grazing-Incidence Small-Angle X-ray Scattering (GISAXS): Used concurrently with GIWAXS to monitor film morphology, grain size evolution, and differentiate between 2D (wetting layer) and 3D (island) growth modes.
  4. Atomic Force Microscopy (AFM) and Fluorescence Microscopy: Used to visualize surface morphology, grain boundaries, and nanofiber alignment, complementing the scattering data.
  5. Substrate Surface Functionalization: Employing Octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) to alter surface energy and promote 2D growth over 3D island formation.
  6. 2D Material Templating: Utilizing Graphene (Type I and Type II) and Molybdenum Disulfide (MoS2) with controlled horizontal or vertical alignment to steer molecular orientation and epitaxy.
  7. High-Pressure Single Crystal X-ray Diffraction: Using Diamond Anvil Cells (DACs) to study phase transitions and map intermolecular interactions (e.g., S…H bonds) under extreme compression (up to 8 GPa).
  8. In Operando Device Characterization: Monitoring the crystalline structure of the active layer within functioning OFET devices under applied gate voltage to assess structural stability during operation.

Commercial Applications

Section titled “Commercial Applications”

The structural control and stability demonstrated in these organic semiconductor thin films are highly relevant for advanced electronic and optoelectronic devices:

  • Organic Field-Effect Transistors (OFETs): Development of high-mobility p-type semiconductors where precise control over molecular orientation (standing-up for optimal charge transport) and structural stability under electrical stress are required.
  • Flexible and Wearable Electronics: Utilizing 2D material substrates (Graphene, MoS2) and low-temperature vacuum deposition techniques to create structurally robust active layers for flexible device architectures.
  • Optoelectronic Devices: Tuning the molecular orientation (edge-on vs. face-on) to optimize light absorption and emission properties for use in Organic Photovoltaics (OPV), photosensors, and Light-Emitting Transistors (LETs).
  • Polarization-Sensitive Sensors: Leveraging the ability to induce highly aligned, crystalline nanofiber structures (e.g., on mica) for directional light detection and sensing applications.
  • Interface Engineering Products: Developing robust recipes for substrate functionalization (e.g., OTS SAMs) to reliably control the nucleation and growth mode of organic films in industrial vacuum processing environments.
View Original Abstract

Abstract Recent structural studies of small‐molecular‐weight thiophenes are surveyed, with particular focus on naphthyl‐end‐capped derivatives and comparison to alkyl‐capped and unsubstituted analogues. Grazing‐incidence wide‐angle X‐ray scattering of 5,5′‐bis(naphth‐2‐yl)‐2,2′‐bithiophene (NaT2) on octadecyl‐trichlorosilane‐passivated Si, graphene, MoS 2 , muscovite mica, and in operando thin‐film transistors reveals substrate‐dependent unit cells, polymorphs, strain fields, and epitaxial orientations. Bulk crystallography exposes multiple polymorphs in ambient conditions and under compression up to the gigapascal regime. In situ vacuum deposition experiments track layer‐by‐layer nucleation, a wetting‐layer-mediated 2D‐to‐3D transition, and the emergence of bulk packing. High stability permits long measurements, whereas strong crystallinity enables high quality diffraction signals even from monolayers and through diamond‐anvil cells and high‐background vacuum chambers. Detailed comparisons with other small‐molecular‐weight thiophenes are made throughout to contextualize and generalize these observations. Together these results establish naphthyl‐terminated thiophenes as convenient model systems for probing substrate interactions, growth modes, and strain‐coupled polymorphism in organic semiconductors.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1999 - Handbook of Oligo‐ and Polythiophenes

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