[Paper Review] Direct observation of ordered configurations of hydrogen adatoms on graphene

Chenfang Lin, Yexin Feng+8 more|arXiv (Cornell University)|Sep 5, 2014

Graphene research and applications3 citations

TL;DR

This study presents the first direct experimental observation of ordered hydrogen adatom configurations on graphene using scanning tunneling microscopy (STM) and spectroscopy (STS). Ordered H adatoms bind preferentially to carbon atoms on the same sublattice, forming configurations with a local density of states gap exceeding 0.6 eV, consistent with density functional theory calculations of double-sided H adsorption structures.

ABSTRACT

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated and searched for. Here we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. A gap larger than 0.6 eV in the local density of states of the configurations was revealed by scanning tunneling spectroscopy measurements. These findings can be well explained by density functional theory calculations based on double sided H configurations. In addition, factors that may influence H ordering are discussed.

Motivation & Objective

• To experimentally confirm long-predicted ordered configurations of hydrogen adatoms on graphene.
• To resolve the atomic-scale structure and electronic properties of hydrogenated graphene regions.
• To identify the underlying bonding and electronic mechanisms responsible for H adatom ordering.
• To investigate factors influencing the formation and stability of ordered H adatom phases on graphene.

Proposed method

• Employed low-temperature scanning tunneling microscopy (STM) to image hydrogen adatoms on suspended graphene membranes.
• Conducted scanning tunneling spectroscopy (STS) to measure the local density of states (LDOS) of H-adsorbed regions.
• Analyzed STM topographs to identify sublattice-specific binding of H adatoms, revealing symmetry in adatom arrangements.
• Used density functional theory (DFT) calculations to model double-sided H configurations and compare predicted LDOS with experimental STS data.
• Correlated observed adatom patterns with theoretical predictions of energetically favorable H adsorption geometries.
• Evaluated the influence of substrate interactions and defects on H adatom ordering through comparative imaging.

Experimental results

Research questions

• RQ1 What atomic-scale configurations do hydrogen adatoms adopt on graphene under controlled conditions?
• RQ2 How do the electronic properties of H-adsorbed graphene regions differ from pristine graphene?
• RQ3 Can ordered H adatom phases be directly observed using STM, and what structural features define them?
• RQ4 What is the role of double-sided H adsorption in stabilizing ordered configurations?
• RQ5 What experimental and environmental factors affect the formation and stability of ordered H adatom phases?

Key findings

• Ordered configurations of hydrogen adatoms were directly observed on graphene using high-resolution STM, with H atoms preferentially binding to carbon atoms on the same sublattice.
• Scanning tunneling spectroscopy revealed a local density of states gap larger than 0.6 eV in H-adsorbed regions, indicating a significant electronic modification.
• The observed structures and electronic properties were well explained by DFT calculations assuming double-sided hydrogen adsorption configurations.
• The ordered H adatom arrangements exhibited long-range periodicity and symmetry consistent with theoretical predictions of sublattice-selective adsorption.
• Factors such as substrate interactions and defects were found to influence the stability and formation of ordered H adatom phases.
• The experimental findings provide direct evidence supporting theoretical models of hydrogenated graphene with tunable electronic band gaps.