A First: Hydrogen Atoms Manipulated Below the Surface of a Palladium Crystal

Simulated “flight” close to and then above the structure shown in the PSU figure (below).

For the first time, scientists have manipulated hydrogen atoms into stable sites beneath the surface of a palladium crystal, creating a structure predicted to be important in metal catalysts, in hydrogen storage, and in fuel cells. The research will be published in the 13 December 2005 issue of the journal Proceedings of the National Academy of Science.

Observations of the effects of the resulting subsurface hydrides--hydrogen atoms with a partial negative charge--confirmed the existence of the stable sites, which had been predicted but previously had neither been deliberately assembled nor directly observed. The research was led by Paul S. Weiss, Distinguished Professor of Chemistry and Physics at Penn State.

After moving absorbed hydrogen atoms to just below the crystal surface, the researchers were able to observe how the presence of the hydride in specific sites within a metal crystal affects the chemical, physical, and electronic properties of the metal. Understanding these effects could advance efforts to improve chemical reactions involving metal catalysts. In addition, the subsurface hydride may provide a model material for application in hydrogen storage and fuel cells. The ability to prepare the subsurface hydride provides an important research tool for these applications.

Weiss points out that hydrogen atoms just below the surface of the metal have been thought to be important in a number of chemical reactions. "Indirect experimental data have shown that chemically reactive hydrogen atoms were located at such sites, but there was no way to test them," says Weiss. "This material will allow us to test the predictions and to apply data from direct observation."

The writing is less than half an Angstrom (0.00005 microns) "tall" off the surface. The characters are less than 500 Angstrom (0.05 microns) high. They were created when Weiss and his team moved hydrogen atoms underneath a palladium surface using a custom-built, ultrastable, low-temperature, scanning tunneling microscope.

The researchers carried out the experiments in a low-temperature scanning tunneling microscope (STM) under ultrahigh vacuum by exposing the crystal to a hydrogen atmosphere. They removed excess hydrogen from the surface by cycles of exposure to heat and oxygen. After the surface had been cleaned, the researchers were able to use electrons from the STM tip to move hydrogen atoms that had been absorbed into the bulk metal up into the stable subsurface sites. As the hydride formed underneath the surface of the material, Weiss and his team observed that the surface of the crystal distorted, the positive charge of palladium atoms above them increased, and interactions occurred with hydrogen atoms on the surface of the palladium crystal. "One of the most interesting aspects of the research was the ability to move atoms beneath the surface," Weiss says. "The observation of the effects of the populated sites, such as surface distortion, confirmed the existence of the stable sites and the theoretical predictions of the physical and electrical properties of the hydrides."

Years ago, Weiss was the first on an IBM team to manipulate xenon atoms on a metal surface. His coworkers later moved atoms to spell out their corporate logo. By extending the ability to manipulate atoms beyond the surface of a material, this research is expected to advance the understanding and control of important chemical reactions in a variety of commercial applications. In addition, this ability has potential as a model system of a technologically important material.

In addition to Weiss, other members of the research team include E. Charles H. Sykes, a former postdoctoral fellow in the Weiss lab who is now an assistant professor of chemistry at Tufts University; Luis C. Fernandez-Torres, a former postdoctoral fellow in the Weiss lab who is now an assistant professor of chemistry at the University of Puerto Rico at Cayey; Sanjini U. Hanayakkara, a Penn State graduate student; Brent A. Mantooth, a former graduate student in the Weiss lab who is now a scientist at SAIC Research and Development Center; and Ryan M. Nevin, a former REU undergraduate in the Weiss lab who is now a graduate student at the University of Wisconsin.

This research was funded by the Air Force Office of Scientific Research, with additional support from the Army Research Office, the National Science Foundation, and the Office of Naval Research.

Manipulation of subsurface hydorgen atoms in palladium by scanning tunneling microscopy to form the subsurface hydride.

(A) STM image (700 Å x700 Å, or 0.070 mµ x 0.070 mµ) of four narrow lines of hydride on a Pd{111} surface at 4 K that were created (or ‘‘written’’) by moving the tip over the surface at elevated scanning tunneling microscope probe tip voltages. The vertical scale of the image is only 0.5 Å (0.00005 mµ).

Experimental details: The writing voltage was constant (Vsample = 0.7 V); the currents used for lines 1–4 were 1, 10, 50, and 150 pA, respectively.

(B) STM image (950 Å x 700 Å, or 0.095 mµ x 0.070 mµ) of five narrow lines of hydride created on a Pd{111} surface at 4 K. The vertical scale of the image is 0.6 Å

(0.00006 mµ).

Experimental details: (Vsample = 0.025 V, Itunnel = 50 pA). The writing current was constant

(It = 50 pA), the voltages used for lines 5–9 were -0.6, -1.0, +1.0, +0.6, and +0.7 V, respectively.

The two Insets are differential conductance images (30 Å x 30 Å) showing atomic resolution on the Pd{111} surface. The hexagonal array of surface palladium atoms over the feature was distorted (Lower Inset) when compared to the hexagonal pattern of the flat Pd{111} surface (Upper Inset).

Experimental details: (Vsample = -0.018 V, Itunnel = 200 pA).

Schematic of the effect of populating subsurface sites with hydrogen atoms from the bulk.

(A) The surface and subsurface (SS) regions are free of hydrogen on the clean crystal, whereas the bulk contains some residual hydrogen. 

(B) After applying a bias pulse with the tip, some of the bulk hydrogen segregates to more stable sites in the subsurface region. The surface palladium atoms relax upwards, changing the local topography, and there is a dipole created that leaves the surface palladium atoms electron deficient with respect to their surroundings. A potential energy diagram (one-dimensional) that depicts the relative stability of a hydrogen atom in each of the three regions is shown above each schematic.

Effect of subsurface hydride on H adatoms on A Paladium Cyrstal (Pd{111}).

(A and B) STM images demonstrating hydrogen segregation from the area over which the tip hovered (circle) (150 Å x 150 Å, Vsample = 0.070 V, Itunnel = 50 pA). 

(C and D) STM images depicting hydrogen overlayer vacancy aggregation in the area over which the tip hovered (200 Å x 200 Å, Vsample = -0.050 V, Itunnel = 100 pA).

Schematics depicting the process through which hydrogen atom segregation occurs. The STM images are taken from Figure 3.

(A and B) At low coverage, subsurface hydride sites are filled, the electronic structure of the overlying surface is changed, and its geometric structure is distorted; Surface hydrogen moves away from the sites above. 

(C and D) At high coverage, subsurface hydride sites are filled, the electronic structure of the overlying surface is changed, and its geometric structure is distorted. Surface hydrogen forms a lower density hydrogen overlayer structure over the subsurface hydride as overlayer hydrogen atoms move away; the surrounding areas retain a denser hydrogen overlayer structures.